GitLab CODEOWNERS: support for default section owner

[rarkins]

Discussed in #29182

^{Originally posted by balihb May 21, 2024}

Tell us more.

Gitlab supports sections in CODEOWNERS and can assign default owners for sections.
https://docs.gitlab.com/ee/user/project/codeowners/index.html#set-default-owner-for-a-section

This section must be refactored to support it:

    // Convert CODEOWNERS file into list of matching rules
    const fileOwnerRules = codeOwnersFile
      .split(newlineRegex)
      // Remove comments
      .map((line) => line.split('#')[0])
      // Remove empty lines
      .map((line) => line.trim())
      .filter(is.nonEmptyString)
      // Extract pattern & usernames
      .map(extractOwnersFromLine);