PR triage: add Z-Community-PR label automatically

We've got more and more PRs from the community (and this is great!).

We want to tag them automatically with the label https://github.com/vector-im/element-android/labels/Z-Community-PR

Today, we add the label manually, but it could be nice if this is done automatically by a GitHub Action.

The rule is:
- If the PR author is not in the team `vector-core` (https://github.com/orgs/vector-im/teams/vector-core): add the label https://github.com/vector-im/element-android/labels/Z-Community-PR when the PR is submitted.

Applicable only to new submitted PR I guess.