This commit adds an `eof()` function for grammars, which is easier to
use than the NUL byte directly. It compiles down to a constraint that
the enclosing production can only reduce at end of input, not to a
shiftable token.
BREAKING CHANGE: Public error types for `tree-sitter-generate` were modified.
Co-authored-by: Will Lillis <will.lillis24@gmail.com>
