src: do proper StringBytes error handling

- Return `MaybeLocal`s from `StringBytes::Encode`
- Add an `error` out parameter to pass JS exceptions to the callers
  (instead of directly throwing)
- Simplify some of the string generation methods in `string_bytes.cc`
  by unifying the `EXTERN_APEX` logic
- Reduce usage of deprecated V8 APIs.
- Remove error handling logic from JS, the `buffer.*Slice()` methods
  now throw errors themselves.
- Left TODO comments for future semver-major error message
  improvements.

This paves the way for better error messages coming out of the
StringBytes methods.

Ref: https://github.com/nodejs/node/issues/3175
PR-URL: https://github.com/nodejs/node/pull/12765


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>