sed: Perform case insensitive substitutions with "/i"

Author: https://github.com/water-ghosts
Commit: https://github.com/SerenityOS/serenity/commit/96d3d3b0fe
Pull-request: https://github.com/SerenityOS/serenity/pull/16584
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/gmta
Reviewed-by: https://github.com/kleinesfilmroellchen
Reviewed-by: https://github.com/rtobar

Userland/Utilities/sed.cpp

@@ -90,6 +90,8 @@ static ErrorOr<SubstitutionCommand> parse_command(StringView command)
         } else if (flag == "g"sv) {
             // Allow multiple matches per line by un-setting the SingleMatch flag
             options &= ~PosixFlags::SingleMatch;
+        } else if (flag == "i"sv || flag == "I"sv) {
+            options |= PosixFlags::Insensitive;
         } else {
             warnln("sed: Unsupported flag: {}", flag);
         }