New command "string pad" to pad text to a given width (#7340)

Pads text to a given width, or the maximum width of all inputs.

tests/checks/string.fish

@@ -42,6 +42,29 @@ string length "hello, world"
 string length -q ""; and echo not zero length; or echo zero length
 # CHECK: zero length
 
+string pad foo
+# CHECK: foo
+
+string pad -r -w 4 foo
+# CHECK:  foo
+
+string pad -r -w 7 -c '-' foo
+# CHECK: foo----
+
+string pad --width 7 -c '=' foo
+# CHECK: ====foo
+
+echo \|(string pad --width 10 --right foo)\|
+# CHECK: |foo       |
+
+string pad -w 4 -c . 🐟
+# CHECK: ..🐟
+
+string pad -c . long longer longest
+# CHECK: ...long
+# CHECK: .longer
+# CHECK: longest
+
 string sub --length 2 abcde
 # CHECK: ab
 
@@ -192,7 +215,7 @@ string unescape --style=url (string escape --style=url 'a b#c"\'d')
 # CHECK: a b#c"'d
 
 string unescape --style=url (string escape --style=url \na\nb%c~d\n)
-# CHECK: 
+# CHECK:
 # CHECK: a
 # CHECK: b%c~d
 
@@ -260,7 +283,7 @@ string replace -a " " _ "spaces to underscores"
 # CHECK: spaces_to_underscores
 
 string replace -r -a "[^\d.]+" " " "0 one two 3.14 four 5x"
-# CHECK: 0 3.14 5 
+# CHECK: 0 3.14 5
 
 string replace -r "(\w+)\s+(\w+)" "\$2 \$1 \$\$" "left right"
 # CHECK: right left $
@@ -295,7 +318,7 @@ and echo Unexpected exit status at line (status --current-line-number)
 # 'string match -r with empty capture groups'
 string match -r '^([ugoa]*)([=+-]?)([rwx]*)$' '=r'
 #CHECK: =r
-#CHECK: 
+#CHECK:
 #CHECK: =
 #CHECK: r
 
@@ -575,13 +598,13 @@ printf '[%s]\n' (string collect one\n\n two\n)
 # CHECK: [two]
 printf '[%s]\n' (string collect -N one\n\n two\n)
 # CHECK: [one
-# CHECK: 
+# CHECK:
 # CHECK: ]
 # CHECK: [two
 # CHECK: ]
 printf '[%s]\n' (string collect --no-trim-newlines one\n\n two\n)
 # CHECK: [one
-# CHECK: 
+# CHECK:
 # CHECK: ]
 # CHECK: [two
 # CHECK: ]