Before this change, Lexer::is_identifier_{start,middle}() would do a
Unicode property lookup via Unicode::code_point_has_property() quite
frequently, especially for common characters like .,;{}[]() etc.
Since these and any other ASCII characters not covered by the alpha /
alphanumeric check are known to not have the ID_Start / ID_Continue
(except '_', which is special-cased now) properties, we can easily
avoid this function call.
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