Curly underlines in set_color and fish_color_*

set_color --underline=curly outputs \e[4:3m which breaks the following
terminals:
- Terminal.app interprets it as yellow background
- abduco and dvtm interpret it as green foreground
- JetBrains terminals interprets it as yellow background
- urxvt interprets it as yellow background

terminals that interpret curly as single underline:
- tmux [1]
- emacs ansi-term [2]
- emacs vterm
- GNU screen (also wrongly turns on italic mode)
- terminology (also wrongly turns on italic mode)
- Vim's :terminal

[1]: https://github.com/orgs/tmux/discussions/4477
[2]: https://lists.gnu.org/archive/html/bug-gnu-emacs/2025-04/msg01093.html

Closes #10957

share/tools/web_config/webconfig.py

@@ -240,7 +240,11 @@ def parse_color(color_str):
         if comp == "--bold" or comp == "-o":
             bold = True
         elif comp == "--underline" or comp == "-u":
-            underline = True
+            underline = "single"
+        elif comp.startswith("--underline="):
+            underline = comp.stripprefix("--underline=")
+        elif comp.startswith("-u"): # Multiple short options like "-rbcurly" are not yet supported.
+            underline = comp.stripprefix("-u")
         elif comp == "--italics" or comp == "-i":
             italics = True
         elif comp == "--dim" or comp == "-d":
@@ -295,8 +299,8 @@ def unparse_color(col):
         ret += col["color"]
     if col["bold"]:
         ret += " --bold"
-    if col["underline"]:
-        ret += " --underline"
+    if col["underline"] is not None:
+        ret += " --underline=" + col["underline"]
     if col["italics"]:
         ret += " --italics"
     if col["dim"]: