Syntax highlighting for CODEOWNERS file in Source Editor

As a first leg in protecting the CODEOWNERS file and supporting the creation of valid files, we should introduce support for this file type in the Source Editor.

This issue is aiming at introducing the first step in this direction as in provide the syntax highlighting to the users. It does not imply validation right away - validation should be handled in a separate issue as it has multiple levels of ~UX involvement.

Implementation suggestion

The task can be implemented by introducing a new extension for Source Editor which would take care of everything CODEOWNERS-related with the highlighting being the first step.