Make isIDFIRST_uni() return identically as isIDFIRST_utf8()

Make isIDFIRST_uni() return identically as isIDFIRST_utf8()

These two macros should have the same results for the same input code
points.  Prior to this patch, the _uni() macro returned the official
Unicode ID_Start property, and the _utf8() macro returned Perl's
slightly restricted definition.  Now both return Perl's.