Make NodePath<T | U> distributive

[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

This pull request makes it so that the types NodePath<t.Identifier | t.StringLiteral> and NodePath<t.Identifier> | NodePath<t.StringLiteral> are equivalent. This makes our path.is* able to better perform type narrowing:

if (path.isIdentifier()) {
  // path is NodePath<t.Identifier> -- this already worked
} else {
  // path is NodePath<t.StringLiteral> -- this works thanks to this PR
}

It also makes #16430 slightly faster, because it reduces the number of possible NodePath<...> types (because NodePath<t.Identifier> | NodePath<t.StringLiteral> and NodePath<t.Identifier | t.StringLiteral> are now the same type).

Unfortunately it makes it more common for .ensureBlock() to not work, since asserts return types do not work on unions (microsoft/TypeScript#44212). I consider this acceptable, since path.is* are much more common.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/56735

[JLHwung]

Does parent path type narrowing still work in this PR?

[nicolo-ribaudo]

Yes, because it's properly defined further down in the NodePath interface.

[liuxingbaoyu]

Amazing!
I've tried something similar before and failed. :)

[JLHwung]

Awesome work.