LibJS: Hook up Regex<ECMA262> to RegExpObject and implement `test()'

This makes RegExpObject compile and store a Regex<ECMA262>, adds
all flag-related properties, and implements `RegExpPrototype.test()`
(complete with 'lastIndex' support) :^)
It should be noted that this only implements `test()' using the builtin
`exec()'.

Libraries/LibRegex/RegexMatch.h

@@ -261,6 +261,7 @@ public:
 struct MatchInput {
     RegexStringView view { nullptr };
     AllOptions regex_options {};
+    size_t start_offset { 0 }; // For Stateful matches, saved and restored from Regex::start_offset.
 
     size_t match_index { 0 };
     size_t line { 0 };