Fix 'treesit-max-buffer-size' and its use

Fix 'treesit-max-buffer-size' and its use

* lisp/treesit.el (treesit-max-buffer-size): Avoid overflow in
computing buffer-size limit.  Account for 32-but systems built
"--with-wide-int".  Extend doc string.
(treesit-ready-p): Compare the limit with the size of the buffer
in bytes, not in characters.

* src/treesit.c (treesit_check_buffer_size): Measure buffer size
in bytes.