Add alias attribute

[marcaddeo]

Implements #2349

[marcaddeo]

@casey how do you feel about this implementation?

[casey]

Just took a first look at this!

I think it's probably not a good idea to try to construct a Name token, when the source was in fact a string literal, and that Alias should be refactored to make the name field a String. If we need a token so we can report error locations, we shoiuld use a Token, and the underlying TokenKind may be a Identifier or a StringLiteral, depending on the source.

You can use Lexer::is_identifier to check if StringLiteral::cooked is a valid identifier. StringLiteral::cooked should be use, since it may contain escape characters.