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.

Libraries/LibGUI/JSSyntaxHighlighter.cpp

@@ -42,6 +42,7 @@ static TextStyle style_for_token_type(Gfx::Palette palette, JS::TokenType type)
     case JS::TokenType::NumericLiteral:
         return { palette.syntax_number() };
     case JS::TokenType::StringLiteral:
+    case JS::TokenType::TemplateLiteral:
     case JS::TokenType::RegexLiteral:
     case JS::TokenType::UnterminatedStringLiteral:
         return { palette.syntax_string() };