webconfig: Replace str.stripprefix with str.removeprefix

str.stripprefix doesn't exist in Python:
https://docs.python.org/3/library/stdtypes.html#str.removeprefix

Closes #11840

CHANGELOG.rst

@@ -8,6 +8,7 @@ This release fixes the following regressions identified in 4.1.0:
 
 - Some :doc:`fish_config <cmds/fish_config>` subcommands for prompts and themes were broken in standalone Linux builds (those using the ``embed-data`` cargo feature), which has been fixed (:issue:`11832`).
 - Our new workaround for WezTerm's `issues with modifyOtherKeys <https://github.com/wezterm/wezterm/issues/6087>`__ breaking shifted keys was broken on some versions of WezTerm, which has been fixed (:issue:`11204`).
+- Fixed a crash when using new underline styles in :doc:`the web-based configuration tool <cmds/fish_config>` (:issue:`11840`).
 
 fish 4.1.0 (released September 27, 2025)
 ========================================

share/tools/web_config/webconfig.py

@@ -243,11 +243,13 @@ def parse_color(color_str):
         elif comp == "--underline" or comp == "-u":
             underline = "single"
         elif comp.startswith("--underline="):
-            underline = comp.stripprefix("--underline=")
+            # TODO(python>3.8): use removeprefix
+            underline = comp[len("--underline=") :]
         elif comp.startswith(
             "-u"
         ):  # Multiple short options like "-rucurly" are not yet supported.
-            underline = comp.stripprefix("-u")
+            # TODO(python>3.8): use removeprefix
+            underline = comp[len("-u") :]
         elif comp == "--italics" or comp == "-i":
             italics = True
         elif comp == "--dim" or comp == "-d":