Summary of the changes:
* define the `exports` field in `package.json`; `jQuery` & `$` are also
exported as named exports in ESM builds now
* declare `"type": "module"` globally except for the `build` folder
* add the `--esm` option to `grunt custom`, generating jQuery as an ECMAScript
module into the `dist-module` folder
* expand `node_smoke_tests` to test the slim & ESM builds and their various
combinations; also, test both jQuery loaded via a path to the file as well
as from module specifiers that should be parsed via the `exports` feature
* add details about ESM usage to the release package README
* run `compare_size` on all built minified files; don't run it anymore on
unminified files where they don't provide lots of value
* remove the remove_map_comment task; SWC doesn't insert the
`//# sourceMappingURL=` pragma by default so there's nothing to strip
Fixes gh-4592
Closes gh-5255
In gh-4466, we removed the `external` directory in favor of loading some files
directly from `node_modules`. This works fine locally but when deploying code
for tests, this makes it impossible to not deploy `node_modules` as well. To
avoid the issue, this change restores usage of the `external` directory.
One change is that we no longer commit this directory to the repository, its
only purpose is to have clear isolation from `node_modules`.
Ref gh-4466
Closess gh-4865
Now that Sizzle is gone & we use npm, we can read from node_modules directly
and skip the setup that copies some files to the external directory.
Closes gh-4466
