Introduce a GitHub Actions workflow for syncing package updates to a branch

[johnbillion]

Syncing Gutenberg package updates back to a WordPress core branch can be done with the npm run sync-gutenberg-packages -- --dist-tag=wp-x.x command, but this can slow down the release of branches, especially when multiple backports are needed. Ideally this could be done ahead of time in a GitHub Actions workflow which allows the build and tests to run to ensure they're passing before backports are started.

A GitHub Actions workflow should be introduced which:

• Accepts a WordPress version number input in major.minor format
• Runs npm install
• Runs npm run sync-gutenberg-packages -- --dist-tag=wp-x.x where x.x is the branch
• Commits and opens a PR against the branch
• Runs the build and the tests

[desrosj]

I'd like to take a look at several of these GitHub Actions related improvements during 7.0. Placing this in the milestone to have a look once 6.9 is released.