Clean up GitHub Actions workflow

[bendrucker]

Fixes some issues from #152

• Duplicate trigger when opening PRs from this repo (no fork)
• Remove Yarn, npm is just fine
• Lint on latest LTS only
• Test always, even if lint fails
• Don't check types on multiple Node versions
• Remove useless actions/checkout option (workflow only has read permissions so persist-credentials: false is unimportant)