Refactor command registration and tree expansion events

[AndrewTwydell]

What It Does
Refactor extension.ts to reduce repetition. The majority of the expansion events were 1 of 2 methods, with a couple of exceptions. The refactor ensures the common behaviour share a single method.

I also moved the commands registration out of the file to it's own method for tidiness.

Review Checklist
I certify that I have:

• tested my changes
• added/updated automated tests
• updated the changelog
• followed the contribution guidelines

[davenice]

I think this looks sensible. I like beginning to split out extension.ts because it will reduce the number of merge conflicts we hit.

I like the map of types to actions.

I've tested this locally, expanding a full tree in a CICSplex and in an SMSS region.

[zFernand0]

I really like how clean the extension.ts file looks after refactoring 🙏

LGTM! 😋