Add prop keySeparator

[PRGfx]

Introduce the option to split/join node paths by a character other than /.

It seems I had to go through the walk setup, which led to a lot of changes in the tests.
I'm not convinced about defining the default separator in two places, but I am not sure where best to define such "shared" value.

Resolves #162

[iannbing]

hi @PRGfx , thanks for the PR! It's indeed a useful prop!
Is it possible to make this test case more specific by setting openNodes to item2/a;b (and b needs nodes as well)? So that it can fully demonstrate the use case of this new prop 🙂

[iannbing]

@PRGfx Thanks for the PR!
Could you improve the test case? I'd be happy to merge it.

[PRGfx]

Hey, sorry for the delay, I don't really need this feature. I adjusted the test-case as you suggested. However openNodes needs to be ['item2/a', 'item2/a;b'] - I would have expected that it would open this path on its own, but that's for another topic.

[iannbing]

Closing this as superseded by the v2 rewrite.

The keySeparator feature shipped as part of v2.0.0 — currently published as react-simple-tree-menu@2.0.0-rc.2 on npm's next tag. Implementation in PR #220, which bundled it with the broader rewrite:

• keySeparator?: string on <TreeMenu> (default "/") — threads through everywhere the library touches path keys: emitted Item.key paths in walk(), openNodes lookups, LEFT-arrow parent-focus navigation, and a matching optional second arg on unflatten(items, keySeparator?).
• New public helper collectBranchKeys(data, keySeparator?) for consumers doing controlled openNodes who want an expand-all that respects the same separator.

Thanks for the original proposal and for flagging issue #162 — the design conversation here helped shape the final prop.

Try the RC:

```
npm install react-simple-tree-menu@next
```