buffer: improve Buffer#fill performance

1) This improves the performance for Buffer#fill by using shortcuts.
2) It also ports throwing errors to JS. That way they contain the
proper error code.
3) Using negative `end` values will from now on result in an error
instead of just doing nothing.
4) Passing in `null` as encoding is from now on accepted as 'utf8'.

PR-URL: https://github.com/nodejs/node/pull/18790


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>