lib: rename js source to lower snake_case

This commit renames all JavaScript source files in lib to lower
snake_case.

PR-URL: https://github.com/nodejs/node/pull/19556


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>