[spec/lex] Optional IdentifierStart after string/numeric literal suffix

[ntrel]

This is for dlang/dmd#15339.

I have ignored the ImaginarySuffix FloatLiteral variants, as they are deprecated.

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[dkorpel]

In the other PR you said:

More importantly, we can never add any new literal suffixes without breaking code if we don't have an error.

When you only put IdentifierStartError after existing suffixes, you still can't add new suffixes without breaking code.

[ntrel]

Yes, this change will only catch existing suffixes running into an identifier. The more general errors could be done afterwards - now there's a way to handle them in the grammar.