Source Editor core cleanup

What does this MR do and why?

This MR cleans up Source Editor core to get rid of the parts that have not been in use, yet unnecessarily overcomplicate the solution. Source Editor is in the middle of a big refactoring at the moment and this step will make the refactoring more straightforward and clean.

What exactly has been changed?

• Removed ability to add extensions with a parameter passed to createInstance. This feature was meant to be used with the Vue component but proved to be buggy at the moment and not used. We will get back to this once a real implementation pops up
• Removed ability to use extensions from the global editor's level. Now, when one needs to use an extension this should be done from the instance level only. Since every instance is different there's hardly any scenario where we need to add the same extension to ALL instances of this global editor.
• Moved notification about instance being ready to createInstance so that it would be universally available. Before this MR, this event has been dispatched only when extensions were passed as an option to createInstance().

Relevant links

• PoC for the refactoring
• Source Editor Extension module
• Introduced the Source Editor Instance module
• Extended tests coverage for SE instance

Screenshots or screen recordings

No visible changes

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)