implement .isEmpty() method

[cdisselkoen]

Description of changes

New builtin .isEmpty() method, which works on Sets. Implemented similarly to .contains() which works on Sets. This is our first builtin method that takes 0 arguments other than its receiver (although we have extension methods that take 0 arguments other than their receiver).

This PR contains the changes necessary in this repo (unless I've missed something); we'll also need changes in DRT, the Lean model, and the public docs.

Issue #, if available

Fixes #1356

Checklist for requesting a review

The change in this PR is (choose one, and delete the other options):

• A backwards-compatible change requiring a minor version bump to cedar-policy (e.g., addition of a new API).

I confirm that this PR (choose one, and delete the other options):

• Updates the "Unreleased" section of the CHANGELOG with a description of my change (required for major/minor version bumps).

I confirm that cedar-spec (choose one, and delete the other options):

• Requires updates, and I have made / will make these updates myself. (Please include in your description a timeline or link to the relevant PR in cedar-spec, and how you have tested that your updates are correct.)

I confirm that docs.cedarpolicy.com (choose one, and delete the other options):

• Requires updates, and I have made / will make these updates myself. (Please include in your description a timeline or link to the relevant PR in cedar-docs. PRs should be targeted at a staging-X.Y branch, not main.)