Add hasInserterItems selector and improve getInserterItems selector performance

[youknowriad]

The getInserterItems is called often (even if the inserter in not open) and its memoization is not performant.

In this PR, I'm adding a new hasInserterItems selector which is more performant as it doesn't have to create inserter items and can return quickly after one insertable item is found.

I also refactored a little bit the selectors to avoid using a memoized version of canInsertBlockType as I think it's just not efficient to call memoized selectors inside other memoized selectors.

I don't have the numbers as it's very hard to measure but I'd love your thoughts. I'm also considering dropping the memoization of these two selectors entirely because the cache is invalidated too often IMO.

[tofumatt]

Explanation makes sense to me and I think the code reads fine. 👍

[aduth]

Anecdotally, I observed in a recent performance audit that this function (getInserterItems) accounted for 20% of total JavaScript time spent while actively typing within a Columns block content:

(Which is to say: This pull request should hopefully help)

[lkraav]

I built and ran this minute's master 4a54cc1 on our sample problematic content, and was not able to detect any user-visible block editing performance improvement yet. Quietly assuming here it requires more pieces to fall in place.