LibJS: Add FIXME to removed SplitMatch AO

Author: https://github.com/linusg
Commit: https://github.com/SerenityOS/serenity/commit/24237ae5bf
Pull-request: https://github.com/SerenityOS/serenity/pull/16273
Reviewed-by: https://github.com/trflynn89 ✅

Userland/Libraries/LibJS/Runtime/StringPrototype.cpp

@@ -46,6 +46,7 @@ static ThrowCompletionOr<Utf16String> utf16_string_from(VM& vm)
 }
 
 // 22.1.3.21.1 SplitMatch ( S, q, R ), https://tc39.es/ecma262/#sec-splitmatch
+// FIXME: This no longer exists in the spec!
 static Optional<size_t> split_match(Utf16View const& haystack, size_t start, Utf16View const& needle)
 {
     auto r = needle.length_in_code_units();