Add `--mangle-props regex="/^_/"` in terser for our core dist files.

bootstrap.min.js: 17.03KB -> 16.35KB (gzip)

For the bundle I'm not sure we should enable it since we need to make sure Popper follows the same scheme.

The gains are pretty big, though. We should really look into enabling this.

• test the minified dist file to make sure everything works

Fixes #23635