feat: add pre-commit hook for validator (#1002)

This will allow users to validate their Forgejo Actions files (both actions and workflows) prior to committing them to their repositories, using a `pre-commit` configuration similar to
```yaml
  - repo: https://code.forgejo.org/forgejo/runner
    rev: v11.1.0
    hooks:
      - id: forgejo-runner-validate
```

<!--start release-notes-assistant-->
<!--URL:https://code.forgejo.org/forgejo/runner-->
- features
  - [PR](https://code.forgejo.org/forgejo/runner/pulls/1002): <!--number 1002 --><!--line 0 --><!--description ZmVhdDogYWRkIHByZS1jb21taXQgaG9vayBmb3IgdmFsaWRhdG9y-->feat: add pre-commit hook for validator<!--description-->
<!--end release-notes-assistant-->

Co-authored-by: Frederik “Freso” S. Olesen <freso.dk@gmail.com>
Reviewed-on: https://code.forgejo.org/forgejo/runner/pulls/1002
Reviewed-by: earl-warren <earl-warren@noreply.code.forgejo.org>
Co-authored-by: Freso <freso@noreply.code.forgejo.org>
Co-committed-by: Freso <freso@noreply.code.forgejo.org>

.pre-commit-hooks.yaml

@@ -0,0 +1,16 @@
+- id: forgejo-runner-validate
+  name: Validate Forgejo Actions files
+  description: This hook validates Forgejo Actions action and workflow files.
+  language: golang
+  entry: runner validate
+  args: ['--directory', '.']
+  pass_filenames: false
+  files: (?:(?:^|/)action|^\.(?:forgejo|github|gitea)/workflows/[^/\n]+)\.ya?ml$
+  types: [yaml]
+  # 3.2.0 is when the pre-* `stages` used here were added.
+  # Old names (without the pre- prefix) are deprecated since 4.0.0.
+  minimum_pre_commit_version: '3.2.0'
+  stages: [pre-commit, pre-merge-commit, pre-push, manual]
+  # validate doesn’t currently provide non-success exit codes,
+  # so falling back to always printing verbose output for now.
+  verbose: true