Feature: YML validation

[joshua-cannon-techlabs]

Since this action only makes sense to run on pushes to the default branch, we don't have good validation/feedback that the YML isn't completely broken until we've already merged the PR.

I nice easy little PR check to validate the teams yml is "valid" for the action would be lovely.

[joshua-cannon-techlabs]

As a workaround I think I'll write me a little validator to do just that, but it'd likely be underneath our existing umbrella testing check.

[rmacklin]

Perhaps we could add a "validate-only" parameter to the action which only parses the teams.yml file but doesn't attempt to sync changes to the GitHub organization. Then users could add a second workflow that runs on the non-default branches which invokes the team-sync action in that "validate-only" mode.

[splitice]

I agree a validate-only parameter would be a good idea. That could run on pull_request only.