Validate change-notes as a PR check - #4116

This PR extends the behavior of the recently added pr-checks/changenotes.mts tool by:

1. Hardcoding the argument as the newly-added unreleased-change-notes directory, as that is the only destination for change-notes.
2. Adding a step in the pr-checks.yml workflow file for executing pr-checks/changenotes.mts in PRs to validate any changenotes in unreleased-change-notes.

This PR also introduces an "empty" top-level directory, unreleased-change-notes, to be the destination of future change-note files. This can be changed if another location is more suitable.

Risk assessment

For internal use only. Please select the risk level of this change:

• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Which use cases does this change impact?

Workflow types:

• N/A

Products:

• N/A

Environments:

• Testing/None - This change does not impact any CodeQL workflows in production.

How did/will you validate this change?

• Unit tests - I am depending on unit test coverage (i.e. tests in .test.ts files).

If something goes wrong after this change is released, what are the mitigation and rollback strategies?

• Development/testing only - This change cannot cause any failures in production.

How will you know if something goes wrong after this change is released?

• N/A

Are there any special considerations for merging or releasing this change?

• No special considerations - This change can be merged at any time.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.