Create GH Action to reliably determine mergeability

[roryabraham]

Details

When getting ready to automerge a PR, we want to first make sure that it's mergeable. However, Github uses a background job to calculate mergeability... from the GH docs:

The value of the mergeable attribute can be true, false, or null. If the value is null, then GitHub has started a background job to compute the mergeability. After giving the job time to complete, resubmit the request. When the job finishes, you will see a non-null value for the mergeable attribute in the response. If mergeable is true, then merge_commit_sha will be the SHA of the test merge commit

So we can't rely on the boolean result of the mergeable attribute to be readily available. This PR creates a custom Github Action to continually poll mergeability of a PR until the result is non-null, and output the end result.

Fixed Issues

Fixes failed workflow run: https://github.com/Expensify/Expensify.cash/runs/2133374703?check_suite_focus=true

Tests

Added automated tests for the new JS action.

Need to test it's usage in the workflow by merging this PR.