LibJS: Parse slashes after reserved identifiers correctly

Previously we were unable to parse code like `yield/2` because `/2`
was parsed as a regex. At the same time `for (a in / b/)` was parsed
as a division.

This is solved by defaulting to division in the lexer, but calling
`force_slash_as_regex()` from the parser whenever an IdentifierName
is parsed as a ReservedWord.

Userland/Libraries/LibJS/Parser.h

@@ -244,6 +244,7 @@ private:
     void expected(char const* what);
     void syntax_error(DeprecatedString const& message, Optional<Position> = {});
     Token consume();
+    Token consume_and_allow_division();
     Token consume_identifier();
     Token consume_identifier_reference();
     Token consume(TokenType type);