Colored underlines in set_color and fish_color_*

Add a new underline-color option to set_color (instead of adding an optional
color argument to --underline); this allows to set the underline color
independently of underline style (line, curly, etc.). I don't think this
flexibility is very important but this approach is probably the least hacky.

Note that there are two variants:
1. \e[58:5:1m
2. \e[58;5;1m

Variant 1 breaks:
breakage from colon-variant for colored underlines
- cool-retro-term makes text blink
- GNU screen (goes into bold mode)
- terminology (goes into bold mode)

Variant 2 would break:
- mintty (Cygwin terminal) -- it enables bold font instead.
- Windows Terminal (where it paints the foreground yellow)
- JetBrains terminals echo the colons instead of consuming them
- putty
- GNU screen (goes into bold mode)
- st
- urxvt
- xterm
- etc.

So choose variant 1.

Closes #11388
Closes #7619

share/completions/set.fish

@@ -127,6 +127,7 @@ complete -c set -n '__fish_seen_argument -s e -l erase; and __fish_seen_argument
 # Color completions
 complete -c set -n '__fish_set_is_color true false' -x -a '(set_color --print-colors)' -d 'text color'
 complete -c set -n '__fish_set_is_color false true' -a '--background=(set_color --print-colors)'
+complete -c set -n '__fish_set_is_color false true' -a '--underline-color=(set_color --print-colors)'
 complete -c set -n '__fish_set_is_color true false' -a --bold -x
 complete -c set -n '__fish_set_is_color true false' -a --dim -x
 complete -c set -n '__fish_set_is_color true false' -a --italics -x

share/completions/set_color.fish

@@ -1,5 +1,6 @@
 complete -c set_color -x -d Color -a '(set_color --print-colors)'
 complete -c set_color -s b -l background -x -a '(set_color --print-colors)' -d "Change background color"
+complete -c set_color -s b -l underline-color -x -a '(set_color --print-colors)' -d "Change underline color"
 complete -c set_color -s o -l bold -d 'Make font bold'
 complete -c set_color -s i -l italics -d Italicise
 complete -c set_color -s d -l dim -d 'Dim text'

share/tools/web_config/webconfig.py

@@ -227,6 +227,7 @@ def parse_color(color_str):
     comps = color_str.split(" ")
     color = ""
     background_color = ""
+    underline_color = ""
     bold = False
     underline = False
     italics = False
@@ -251,37 +252,41 @@ def parse_color(color_str):
             dim = True
         elif comp == "--reverse" or comp == "-r":
             reverse = True
-        elif comp.startswith("--background"):
-            # Background color
-            c = comp[len("--background=") :]
-            parsed_c = parse_one_color(c)
-            # We prefer the unparsed version - if it says "brgreen", we use brgreen,
-            # instead of 00ff00
-            if better_color(background_color, parsed_c) == parsed_c:
-                background_color = c
-        elif comp.startswith("-b"):
-            # Background color in short.
-            if comp == "-b":
-                if i + 1 == len(comps):
-                    c = ""
-                else:
-                    c = comps[i + 1]
-                    i += 1
-            else:
-                c = comp[len("-b"):]
-            parsed_c = parse_one_color(c)
-            if better_color(background_color, parsed_c) == parsed_c:
-                background_color = c
         else:
-            # Regular color
-            parsed_c = parse_one_color(comp)
-            if better_color(color, parsed_c) == parsed_c:
-                color = comp
+            def parse_opt(current_best: str, i: int, long_opt: str, short_opt: str | None) -> str:
+                if comp.startswith(long_opt):
+                    c = comp[len(long_opt) :]
+                    parsed_c = parse_one_color(c)
+                    # We prefer the unparsed version - if it says "brgreen", we use brgreen,
+                    # instead of 00ff00
+                    if better_color(current_best, parsed_c) == parsed_c:
+                        return True, c, i
+                elif short_opt is not None and comp.startswith(short_opt):
+                    if comp == short_opt:
+                        if i + 1 == len(comps):
+                            c = ""
+                        else:
+                            c = comps[i + 1]
+                            i += 1
+                    else:
+                        c = comp[len(short_opt):]
+                    parsed_c = parse_one_color(c)
+                    if better_color(current_best, parsed_c) == parsed_c:
+                        return True, c, i
+                return False, current_best, i
+            is_bg, background_color, i = parse_opt(background_color, i, "--background", "-b")
+            is_ul, underline_color, i = parse_opt(underline_color, i, "--underline-color", None)
+            if not (is_bg or is_ul):
+                # Regular color
+                parsed_c = parse_one_color(comp)
+                if better_color(color, parsed_c) == parsed_c:
+                    color = comp
         i += 1
 
     return {
         "color": color,
         "background": background_color,
+        "underline-color": underline_color,
         "bold": bold,
         "underline": underline,
         "italics": italics,
@@ -309,6 +314,8 @@ def unparse_color(col):
         ret += " --reverse"
     if col["background"]:
         ret += " --background=" + col["background"]
+    if col["underline-color"]:
+        ret += " --underline-color=" + col["underline-color"]
     return ret