Check whether a UTF-16 code unit is a letter in identifier position.
ASCII is answered from the pre-computed table; everything else falls back to
a Unicode letter test.
The parameter is a code unit, not a code point, because the tokenizer
advances with charCodeAt. Two consequences follow, and both match Spring's
char-based tokenizer: a surrogate half of an astral-plane character is not
a letter, and a value above the BMP is not a letter either.
Check whether a UTF-16 code unit is a letter in identifier position.
ASCII is answered from the pre-computed table; everything else falls back to a Unicode letter test.
The parameter is a code unit, not a code point, because the tokenizer advances with
charCodeAt. Two consequences follow, and both match Spring'schar-based tokenizer: a surrogate half of an astral-plane character is not a letter, and a value above the BMP is not a letter either.