Patrick/sync snippet action

[patrick-fs]

Bring in the GitHub action code from my fork: https://github.com/patrick-fs/fullstory-browser-sdk

3 scenarios

Review the output of the Sync snippet step:

• Remote snippet changes are captured in a new PR
• An open PR for snippet synchronization causes the GH Action to fail
• No changes

Caveat

Sorry for the ugly commit history! It's a result of many rounds of testing on my fork as I debugged version management using the octokit SDK. It will be squashed away...