LibJS: Lex and parse regex literals, add RegExp objects

This adds regex parsing/lexing, as well as a relatively empty
RegExpObject. The purpose of this patch is to allow the engine to not
get hung up on parsing regexes. This will aid in finding new syntax
errors (say, from google or twitter) without having to replace all of
their regexes first!

Libraries/LibJS/MarkupGenerator.cpp

@@ -223,6 +223,7 @@ MarkupGenerator::StyleType MarkupGenerator::style_type_for_token(Token token)
     case TokenType::TemplateLiteralEnd:
     case TokenType::TemplateLiteralString:
     case TokenType::RegexLiteral:
+    case TokenType::RegexFlags:
     case TokenType::UnterminatedStringLiteral:
         return StyleType::String;
     case TokenType::BracketClose: