Add release actions

[marcustyphoon]

This creates a Github action that, when manually run after a version bump, submits the extension for Firefox review and signing and (if the prior step doesn't time out) drafts a prefilled Github release with the returned signed xpi file attached, pointing to a (nonexistent-until-published) tag named for the new version.

This also creates a Github action that, when manually run, downloads the latest signed xpi file from Mozilla, checks that it corresponds to the current version, and drafts the same Github release as above if so. In the case where Mozilla's review takes too long and the former action times out, this can do the job of the rest of it.

It should be quite simple to add Chrome web store upload to these using chrome-webstore-upload-cli; it just requires some API keys. I'm not sure how the timeline works, though, re: upload-vs-review-vs-publish.

Note that:

• This isn't fully tested; there's only so much you can do without proper API access. Example result with some mocked APIs: https://github.com/marcustyphoon/XKit/releases/tag/v7.10.1
• I don't know if the XPI files that get returned from AMO have UUIDs in the names. If so, this needs a rename step.
• Version bumping web-ext probably breaks the other web-ext action.

[marcustyphoon]

Yeah, I'm not a big fan either. It seemed like a nice alternative to some bash oneliner stuff I was doing at one point (cat manifest.json | jq -r '.version') but it became not-really-close-to-a-oneliner. We can maybe pop it out into a js file later.