Create action to automatically migrate web-features tags to BCD

[queengooborg]

This PR adds a GitHub Actions workflow that will run the BCD tags migration script automatically and create a pull request in BCD.

Note: for this action to function, a token for a bot account that has access to BCD must be obtained and provided.

[ddbeck]

Thanks for putting this together, Vinyl. Here's what I've learned:

I'm fairly certain that this workflow should reside in the BCD repository. For one, I don't love the idea of having privileged access to BCD shared into web-features. And another is that the absolute timing and frequency of generated PRs seems like it should be under the control of BCD maintainers (e.g., to match reviewer availability), not web-features'.

It might be good to have another look at #1672 (comment) to see what I supposed the workflow for migrating tags into BCD would look like as a routine, ongoing matter.

All that said, I might've missed something here and would love to know if there are compelling reasons to run this workflow out of web-features, apart from the existence of the migrate script here.

[Elchi3]

Per #1672 this should live in BCD and I think it would be nice if it gets triggered whenever dependabot updates web-features for BCD.