Add workflow for code scanning using CodeQL

[geofflamrock]

This PR adds a workflow to perform code scanning using CodeQL. It was generated from the settings area of the repo, and adjusted to remove a few bits and pieces that don't apply to this action.

The workflow will run on pushes to main, PRs and weekly on a Thursday.

An example run of this workflow is here. See also the helpful comment further down from the github bot explaining what is happening :)

Fixes #118

[changeset-bot]

⚠️ No Changeset found

Latest commit: c598941

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[github-advanced-security]

This pull request sets up GitHub code scanning for this repository. Once the scans have completed and the checks have passed, the analysis results for this pull request branch will appear on this overview. Once you merge this pull request, the 'Security' tab will show more code scanning analysis results (for example, for the default branch). Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results. For more information about GitHub code scanning, check out the documentation.