…exports

In addition to the existing `exports` conditional exports we ship
as part of APF v13, we want to expose the non-bundled ESM2020 output
through the `esm2020` conditional name. Additionally we will expose
the flat ES2020 files through the `es2020` conditional field, allowing
consumers (like the CLI) to prioritize certain formats.

e.g. consider the case with RXJS where it currently defaults to
the ESM5 output. The CLI could now set `es2015` as the conditional
to leverage the ES2015 output of RXJS. This unveils a problem though
since this would also mean that `ES2015` output of the framework Angular
packages would be used instead of the available ES2020 output. Here is
the additional `es2020` conditional helpful as it allows the CLI to
prioritize `es2020`, fallback to `es2015` and lastly fallback to `default`.
if none do match for a certain package.

This commit updates the `node` engines range for all Angular
framework packages to:

* No longer support NodeJS v12 `< 12.20`. This is done because APF v13
  uses package export patterns which are only supported as of v12.20.
  https://nodejs.org/api/packages.html#packages_subpath_patterns.

* Allows for the latest v16 NodeJS versions. This matches with the CLI
  which added NodeJS v16 support with angular/angular-cli#21854.

  We already limit this to `>= v16.10.0` in preparation to only
  supporting the LTS minors of Node v16.

BREAKING CHANGE: NodeJS versions older than `v12.20.0` are no longer
supported due to the Angular packages using the NodeJS package exports
feature with subpath patterns.