process: refactor nextTick for clarity

Do not share unnecessary information about nextTick state
between JS & C++, instead only track whether a nextTick
is scheduled or not.

Turn nextTickQueue into an Object instead of a class
since multiple instances are never created.

Other assorted refinements and refactoring.

PR-URL: https://github.com/nodejs/node/pull/17738


Reviewed-By: Anna Henningsen <anna@addaleax.net>