CODEOWNERS editor and validation tool

Proposal

CODEOWNERS editor inspired by the Pipeline editor.

This page would include advanced validations including possibly showing which line was causing an error.

Validations

1. Notify when a conflicting rule has been created
2. Notify when rules are created for files/paths that don't exist
3. Notify of syntax problems in the code owners file

Via @dskim_gitlab

It'd be great if we had a script that checks and verifies the format of codeowner file. It'd be great to be able to see the parsed rules before pushing it out.

See epic: &4498 (closed)

Lint/verify that the CODEOWNERS file syntax/format is valid, similar to how we verify a .gitlab-ci.yml file:

Possible tests:

• Marking approvers that are users of GitLab but not eligible for approving because they are not members of the project
• Marking entries that are valid usernames or emails but not found on the GitLab instance
• Mark what part of a line is used as the path pattern, and what is ignored.