Introduced the Source Editor Instance module

What does this MR do and why?

The MR introduces a new separate module for Source Editor Instance. This is part of a bigger effort for refactoring the architecture of Source Editor and the extensions introduced in the dedicated PoC:

1. Introduce a separate module for Source Editor extensions (merged)
2. Introduce a separate module for Source Editor instance (this MR)
3. Switch existing Source Editor architecture to use the new modules
   • update source_editor.js to replace the current instance and extension implementations with the new one
   • update all existing extensions to follow the introduced changes

Even though we do not introduce this and previous steps behind a feature flag, there are no breaking changes at this stage because the new modules are not in use.

Screenshots or screen recordings

No visual changes

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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 #292943 (closed)