LibJS: Detect invalid unicode and stop lexing at that point

Previously we might swallow invalid unicode point which would skip valid
ascii characters. This could be dangerous as we might skip a '"' thus
not closing a string where we should.
This might have been exploitable as it would not have been clear what
code gets executed when looking at a script.

Another approach to this would be simply replacing all invalid
characters with the replacement character (this is what v8 does). But
our lexer and parser are currently not set up for such a change.

Userland/Libraries/LibJS/Lexer.h

@@ -77,6 +77,8 @@ private:
 
     bool m_allow_html_comments { true };
 
+    Optional<size_t> m_hit_invalid_unicode;
+
     static HashMap<FlyString, TokenType> s_keywords;
     static HashMap<String, TokenType> s_three_char_tokens;
     static HashMap<String, TokenType> s_two_char_tokens;