Add artifact-id output to v4-beta

[konradpabjan]

Part of https://github.com/github/actions-results-team/issues/1987

The upcoming v2.0.0 NPM packages already returns the ID of the artifact that was just created/uploaded.

https://github.com/actions/toolkit/blob/fe3e7ce9a7f995d29d1fcfd226a32bca407f9dc8/packages/artifact/src/internal/shared/interfaces.ts#L17-L21

We just need to be expose this output in this action.

Tested with this example run: https://github.com/bbq-beets/testing-artifacts-v4/actions/runs/6645936116/job/18058310425

Also I'm doing some other misc changes

• Bump node runtime from 16 to 20. 16 will start deprecation soonish so we should just use the latest version
• Doc changes, there are still a lot to go but I'm starting to make some changes