Adding sync-labels workflow for sharing one set of labels across all repos.

[aentinger]

This fixes #41.

On every push that changes relevant files, and periodically, use github-label-sync to configure the repository's issue/PR labels according to the universal, shared, and local label configuration files.

The new labels used a standardized prefixed format that causes all the labels of a given class to be listed together in GitHub's alphabetically sorted labels menu.

Example of a repository that has the workflow applied:

• https://github.com/arduino/arduino-examples/labels
• https://github.com/arduino/arduino-examples/actions/workflows/sync-labels.yml