Bundle standalone using rollup

[JLHwung]

Q	A
Fixed Issues?	Shrink *-standalone size
Tests Added + Pass?	Nope
Documentation PR Link
Any Dependency Changes?	Rollup plugins are added, webpack plugins/gulp-uglify are removed.
License	MIT

This PR supersedes #7581 and is loosely based on #7581 because I find it too difficult to rebase.

This PR translates the webpack config in the language of rollup config. gulp-uglify is also replaced by rollup-plugin-terser.

Updates:
There is a fatal error on the previous size comparison table, some of the dependencies of babel-preset-env-standalone was not bundled due to the importer is not an ES Module. The new size comparison has reflected on this fix. Thanks to @nicolo-ribaudo who fist discovered this error.

Here is the size comparison between Circle CI Artifacts and *-standalone 7.7.4 (babel, env)

	Previous Size	New Size	Change
babel.js	3.04 MB	2.6 MB	-14%
babel.min.js	1.63 MB	1.2 MB	-26%
babel-preset-env.js	2.93 MB	3.3 MB	+13%
babel-preset-env.min.js	1.7 MB	1.3 MB	-24%

The increased size in babel-preset-env.js is from the fact that rollup does not strip the comments by default. For production usage, please refer to babel-preset-env.min.js.

Here is the size comparison between [Circle CI Artifacts](https://app.circleci.com/jobs/github/babel/babel/13068) and `*-standalone` 7.7.4 ([babel](https://unpkg.com/browse/@babel/standalone@7.7.4/), [env](https://unpkg.com/browse/@babel/preset-env-standalone@7.7.4/))

	Previous Size	New Size	Change
babel.js	3.04 MB	2.6 MB	14%
babel.min.js	1.63 MB	1.2 MB	26%
babel-preset-env.js	2.93 MB	1.0 MB	66%
babel-preset-env.min.js	1.7 MB	450 kB	74%

(It seems that buildsize does not comment for unknown reasons...)

[JLHwung]

They are removed since rollup-plugin-babel can combine the helpers into a single block.

[nicolo-ribaudo]

CircleCI is failing because of core-js 3.4.6. It's fixed in 3.4.7, but I don't know how to clear the cache before triggering a build.