src: add node_encoding.cc

Continue moving bits of code out of node.cc ... add node_encoding.cc
as a home for `ParseEncoding` and related functions.

PR-URL: https://github.com/nodejs/node/pull/21112


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>