use unicode character categories to classify identifier characters

fixes #6797, fixes #5936
allow categories So and Me in identifiers
remove special latin-1 identifier char rule (mostly subsumed by adding So)

add category No for identifier continue chars
fix silly mistake in return value of new identifier-char predicates

fix strings test to recognize changed error for invalid identifiers
linalg test fix: unicode combining characters need to come after main character
Merge pull request #6805 from JuliaLang/jb/unicode_identifiers

RFC: use unicode character categories to classify identifier characters