[perl #74022] Parser hangs on some Unicode characters

[perl #74022] Parser hangs on some Unicode characters

This changes the definition of isIDFIRST_utf8 to avoid any characters
that would put the parser in a loop.

isIDFIRST_utf8 is used all over the place in toke.c. Almost every
instance is followed by a call to S_scan_word. S_scan_word is only
called when it is known that there is a word to scan.

What was happening was that isIDFIRST_utf8 would accept a character,
but S_scan_word in toke.t would then reject it, as it was using
is_utf8_alnum, resulting in an infinite number of zero-length
identifiers.

Another possible solution was to change S_scan_word to use
isIDFIRST_utf8 or similar, but that has back-compatibility problems,
as it stops q·foo· from being a strings and makes it an identi-
fier instead.