Improvements to GDScript identifier tokenization

[AThousandShips]

Makes potential_identifier() exit early if not ascii-only.

Edit: Was unsure if the token being created before pushing errors was important, so kept that order.

[adamscott]

Pushed to 4.2 milestone, as this is an enhancement PR and as we're in 4.1 feature freeze. Though, as it seems to be a good one, I added it to the list of PRs to team review to not forget it.

[YuriSizov]

Thanks!

[AThousandShips]

Thank you!