Navigation Block - refactor edit function definition

[getdave]

...the edit function seems to be getting a bit big, I think it'd be nice if it were moved out and given a JSDoc description.

Originally posted by @talldan in https://github.com/_render_node/MDE3OlB1bGxSZXF1ZXN0UmV2aWV3NDI0MjE0OTA5/pull_request_reviews/more_threads

See also:

I wouldn't say it's essential to do all refactoring in this PR, but it is, say ..., important to follow up quick enough :) so we don't forget it or even more, complicate it even further.

#18869 (comment)

---

The Problem

Essentially over time the edit function def has grown organically and it's now being unwieldy. Once #18869 is merged then this problem will become worse. Let's take a moment to refactor it to make it easier to work with and reason about.

Due to the complexity of the block and the rapid development currently underway let's try to break these out into tasks with 1 PR per task to keep it simple to review:

• Extract the block Placeholder to its own component.
• Extract rendered Block UI to its own component.
• Create a utility for creating link blocks and then extract buildNavLinkBlocksFromMenuItems and buildNavLinkBlocksFromPages to utilise this.
• Make handleCreate pure-er by providing arguments directly (eg: selectedCreateActionOption, hasPages...etc).

cc @talldan @draganescu