Require successful build in Gitpod to merge a pull request

The current automatic checks allow changes to be merged even if building the extension fails in Gitpod. This means changes that break the Gitpod build can go unnoticed for a while. Since some devs rely on Gitpod to develop and test code and since it's always easiest to fix a problem right when it's caused, not noticing these problems immediately can delay development work and reviews.

Current checks:

## Implementation

Since checks seem to be defined by branch protection rules ([github documentation](https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/defining-the-mergeability-of-pull-requests/managing-a-branch-protection-rule)) in the project's settings, these changes would have to be done by an admin. There aren't many examples of protection rules so it's not clear whether it's possible to create a rule that requires compilation in Gitpod. This requires more investigation.

Start with the repository's branch protection settings and the linked GitHub documentation, then investigate how the Gitpod extension build is exposed as an automatic check. Confirm whether a rule can require that build to succeed before merging. Done means pull requests cannot merge when the Gitpod build fails.

Written by the indexing model from the issue text.