Editor Lite: support creation of a DiffInstance

What does this MR do?

Provides functionality for creating a Diff Instance in addition to the simple Code Instance. This MR is part of a bigger effort of replacing the custom one-off editor implementation in WebIDE with Editor Lite: #292498 (closed)

⚠ Note that this MR also refactors the Jest spec a bit for it to be closer to the integration rather than pure utility tests.

Screenshots (strongly suggested)

	With this MR
Single-file editor
Snippet

As this MR moves us towards integrating Editor Lite into WebIDE, here's how it looks like when the last bits from the relevant PoC are added on top of this MR.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Relates to #297383 (closed), #292498 (closed)