Support publish workflow when ignore option is set

[augustjk]

Fixes #241

The current implementation of using readChangesetState() on entry is based on @changesets/read which simply reads all of the markdown files in .changeset directory.

Trigging the action while using the ignore option with changeset files present for both ignored and non-ignored packages, the version will run and consume changesets made for non-ignored packages, but leave those of ignored packages. When the PR is merged and the action is run again in this state, instead of moving on to the publish flow, it still enters the version flow and tries to make a PR, which fails as there are no releasable packages in this state.

The fix incorporates @changesets/get-release-plan which actually returns a list of releases which takes into account the ignored packages config. The flow control now utilizes this list to determine whether there's anything to version or publish.

I've done some testing of this change at https://github.com/augustjk/changesets-test-project/actions and confirmed publishing with ignored packages works.

Open questions:

• I've created the variable as hasReleases for the control to better reflect this, and added to the output. The hasChangesets variable and output no longer really serve a purpose within the code. I've left it here so as not to break users currently using this output as advertised on the README. Is this acceptable?
• I'm unsure whether this should be a minor or patch bump. Leaving the output above should keep this non-breaking, but using the count of releases instead of changesets might have some other consequences that I'm not thinking of.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 08128b4

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[s0]

This PR is a little old now, going to try and review / revive it to see if the fix is still valid.