GitHub Action for synchronizing snippet updates

[patrick-fs]

Hello! The first draft of GitHub Actions for synchronizing snippets is ready.

The use case for this PR is here: https://github.com/fullstorydev/fullstory-browser-sdk/issues/14

3 test scenarios

(Using the GitHub Action results on my fork: https://github.com/patrick-fs/fullstory-browser-sdk)

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 to snippet when GH Action is executed

Files of particular importance

• .github/workflows/main.yml defines the Action workflow
• .github/actions/sync-snippet-action/main.js is the script that executes syncronization
• MAINTAINERS.json is used to attach assignees to automatically-generated PRs
• some small tweaks have been made to src/snippet.js but prior changes were reviewed by @bddean

Etc.

• I'm currently using a fake snippet service as part of the workflow. There is a separate arc of work afoot that will build the real endpoint. Once done, the GH Action workflow config will need to be updated with the proper endpoint.
• GitHub Actions execute from master, which is why I needed to clone the fullstorydev/fullstory-browser-sdk repo into my own account to build the action (no branch restrictions on master there).

Cc

@jameremo @marktisham