feat: adds support for user-less rules in sections with default owners

[sverweij]

Description

• adds support for user-less rules in sections with default owners in the parser
• adds support for 'inherited' default owners in the labeler generator
• refactors the (virtual) codeowners parser a bit to make it easier to maintain
• drive-by: removes the src/virtual-code-owners/virtual-teams.schema.ts. It's used nowhere now the validations are done by ryo instead of by ajv.

Motivation and Context

In GitLab CODEOWNERS it's possible to assign default owners in sections, and put no owners in rules in that section:

[some section] @team-gitlab @maintainers
src/important-stuff
tools/
.gitlab/

The rules src/important-stuff, tools/ and .gitlab will then 'inherit' the
owners declared in the section heading (in this case @team-gitlab and @maintainers).

Before this PR these rules would be labeled as unknown rules and treated as erroneous lines. After this PR they're treated as 'normal' rules, but with inherited users.

I haven't seen this in the GitHub documentation yet, but CODEOWNER files that use this construct aren't flagged as illegal either, so it might already work and/ or be a future feature there.

How Has This Been Tested?

• green ci
• additional and updated automated non-regression tests

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Documentation only change
• Refactor (non-breaking change which fixes an issue without changing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• 📖

  • My change doesn't require a documentation update, or ...
  • it does and I have updated it

• ⚖️

  • The contribution will be subject to The MIT license, and I'm OK with that.
  • The contribution is my own original work.
  • I am ok with the stuff in CONTRIBUTING.md.