Align identifier rules with Perl XID_* and tighten whitespace+sigil parsing

[fglock]

This PR makes progress on perl5_t/t/uni/variables.t.

Changes:

• Lexer: use ICU XID_START/XID_CONTINUE for identifier tokenization.
• IdentifierParser: treat evalbytes as bytes (ignore use utf8 hint there for identifier validation), always reject C1 controls as identifier starts, and make $ (e.g. "$\t = 4") a syntax error instead of a punctuation special variable.

Status:

• make: passing
• perl5_t/t/uni/variables.t: 72 failures (down from 76)