add github action

[mmkal]

Fixes #225

@Aslemammad tried out the simplest thing I could think of and it seems to work. Example usage (note that pkg-pr-new is no longer installed in package.json etc.): mmkal/expect-type#146

What would be needed for this to be merge-worthy:

• Docs
• Some idea of release strategy - github actions should be semver'd, which could work nicely since this is an npm package too, but then you'd want to tie the version of the action being used to the npm package installed, rather than assume latest which could be confusing
• Decide if args is good enough for the CLI args. I went for the second option I suggested in GitHub Action? #225 (comment) since it adds basically no maintenance overhead

I do think it makes for a nicer experience. pkg-pr-new should really be only used from a CI runner, right? So it seems like it's good signal to people to use it that way. uses: stackblit-labs/pkg.pr.new... gives more confidence than npx -y pkg-pr-new@latest ... I reckon. Plus tools like renovate will automatically upgrade it this way.