LibJS: Add TokenType::TemplateLiteral

This is required for template literals - we're not quite there yet, but at
least the parser can now tell us when this token is encountered -
currently this yields "Unexpected token Invalid". Not really helpful.

The character is a "backtick", but as we already have
TokenType::{StringLiteral,RegexLiteral} this seemed like a fitting name.

This also enables syntax highlighting for template literals in the js
REPL and LibGUI's JSSyntaxHighlighter.

Userland/js.cpp

@@ -453,6 +453,7 @@ int main(int argc, char** argv)
                     stylize({ start, end }, { Line::Style::Foreground(Line::Style::Color::Magenta) });
                     break;
                 case JS::TokenType::StringLiteral:
+                case JS::TokenType::TemplateLiteral:
                 case JS::TokenType::RegexLiteral:
                 case JS::TokenType::UnterminatedStringLiteral:
                     stylize({ start, end }, { Line::Style::Foreground(Line::Style::Color::Green), Line::Style::Bold });