Move lib/internal/assert.js to lib/internal/assert/assertion_error.js.
This is in preparation for making lib/internal/assert.js a tiny module
for use in Node.js built-ins so that we can use `assert()` without
having to load the entire ~1200 line `assert` module.
PR-URL: https://github.com/nodejs/node/pull/25956
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
