Clean up identifier char handling in keyword lookahead functions

[adams85]

Since ES6, identifiers can also contain astral Unicode code points.

This PR adjusts isAsyncFunction and isUsingKeyword to handle this correctly.

Also, removes redundant check for backslash in isLet (as potential backslash is already tokenized as part of an identifier token).

UPDATE: I just realized that isIdentifierChar checks for astral characters when the astral argument is omitted. So this PR just cleans up and makes things more consistent.

[marijnh]

Thanks! The astral parameter to isIdentifierChar already defaults to true. Could you simplify this by removing those parameters again?

[adams85]

Sure thing!

[marijnh]

Thanks!