Extended the syntax highlighting for CODEOWNERS

What does this MR do and why?

The MR adds support for highlighting the following information in the CODEOWNERS files:

• the optional approval (^ in front of the section's name), and
• the number of required approvals ([n] following the section's name, where n is the number of required approvals)

As requested in !114868 (comment 1322204566)

Screenshots or screen recordings

Theme	Result
Light
Dark
Solarized Light
Solarized Dark
Monokai
None

How to set up and validate locally

Either open an existing CODEOWNERS file and check the highlighting, or create a new one and try adding content. For simplicity and speed, you can paste the following snippet:

# When adding a group as a code owner, make sure to invite the group to the
# project here: https://gitlab.com/gitlab-org/gitlab/-/project_members
# As described in https://docs.gitlab.com/ee/user/project/code_owners.html

* @gitlab-org/maintainers/rails-backend @gitlab-org/maintainers/frontend @gitlab-org/maintainers/database @gl-quality/qe-maintainers @gl-quality/tooling-maintainers @gitlab-org/delivery @gitlab-org/maintainers/cicd-templates @nolith @jacobvosmaer-gitlab @gitlab-org/tw-leadership

^[Source code editing]
.solargraph.yml.example @igor.drozdov

^[Backend][2]
*.rb @gitlab-org/maintainers/rails-backend
*.rake @gitlab-org/maintainers/rails-backend

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #393949 (closed)