Generate helper metadata at build time

[nicolo-ribaudo]

Q	A
Fixed Issues?	Fixes #1, Fixes #2
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

The goal of this PR is to:

• move as much analysis of helpers as possible to build time rather than runtime
• make helpers lighter, by working as much as possible with them as an AST rather than a NodePath at runtime
• get rid of the File class that needs to be injected from @babel/core to @babel/helpers through .ensure(name, FileClass)

This PR reduces the time needed to process a single helper by half (allHelpersNames.map(name => this.addHelper(name)); takes 100ms rather than 190ms on my machine)

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/57012

[liuxingbaoyu]

Great! I like this!

[nicolo-ribaudo]

Support for this is needed in #16510

[nicolo-ribaudo]

The helpers-generated.ts size is increased by 50%, and only by 20% when using gzip (it doesn't really matter because it's mostly a Node.js package, but it's still good to know the tradeoff)

	**Before **	After
Size	80kB	120kB
gzip	20kB	24kB
minified	74kB	100kB
minified+gzip	19kB	22kB