src: remove calls to deprecated v8 functions (NewFromUtf8)

Remove all calls to deprecated v8 functions (here: String::NewFromUtf8) inside
the code (src directory only).

PR-URL: https://github.com/nodejs/node/pull/21926


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>