Add primer-migrate script

[shawnbot]

I went 'round and around on this, and I think having a script that's included with the npm package makes things a lot easier for folks. Here's how it works (from the docs):

primer-migrate

You can also use the included primer-migrate script:

npx -p @primer/css primer-migrate path/to/**/*.scss

The script uses Node.js native APIs only, so there are still zero production/runtime dependencies!

[shawnbot]

FYI, I've tested this on github/github with:

npx @primer/css@0.0.0-5a70749 primer-migrate app/assets/stylesheets/**/*.scss

and it worked like a charm! ✨