ci: test that extension icons are up-to-date

Fixes: gitlab-org/editor-extensions/meta#3 (closed)

This MR adds a CI job that ensures that the icons are up-to-date with icons in https://gitlab.com/gitlab-org/editor-extensions/gitlab-ide-icons

This job runs only on main (unless the MR changes icons). This is the same as our "check-ci-variables" jobs and the reason why it runs only on main is not to confuse contributors with failing that is unrelated to their MR.

The job does these steps:

1. Checks that the repository is not dirty (git status returns empty string)
2. Replaces all existing icons with latest icons from https://gitlab.com/gitlab-org/editor-extensions/gitlab-ide-icons
3. runs git status again to see if any icons changed

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Job output examples

• Example of failure in CI: https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/jobs/4662878873
  • in this run I commented out the CI setting that will disable the job for non-main branches
• Here is pipeline that didn't trigger the job because icons haven't changed https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/pipelines/932754187
• The latest job https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/jobs/4662908601 proves that:
  • the updated icons are up-to-date
  • the job is also run on MRs when we change icons