Add a PR Script Workflow

[blink1073]

Adds a workflow to run a script against a PR as a maintainer. The workflow is meant to be run from a fork using the maintainer's own GitHub credentials.

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[jtpio]

Thanks!

[jtpio]

Similar to this we could also have a workflow that takes an arbitrary command and opens a new PR with the changes (could be under the maintainer GitHub name).

For example this would be useful to update dependencies with commands like jlpm run update:dependency --regex @lumino/ latest

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[blink1073]

I opened #5 to track that idea