Materialize query pipeline to a store and query it

[samwillis]

@KyleAMathews, some quick notes on this:

This is a work in progress draft of how we could do an in-memory "store", materialise to them, and allow then to be queryable with D2.

The fruit-processed.ts example has been updated to use it.

Store is a Map like store, but that emits change events.

You can materialise any keyed D2 pipeline to a Store with Store.materialize, this is a static method on the store rather than the S2 pipeline builder so that it is optional and not included if a user doesn't need it. (they may be materialising to anything)

A Store, or multiple, can be queried with the Store.queryAll static method. You pass a list of stores, and a callback, which receives a D2 pipeline builder.

Mutations to a store can be grouped together with store.transaction, the changes are only emitted at the end of the transaction.

A few (initial) things that I want to change:

• I'm subclassing EventTarget, but this doesn't have nice typing of events.
• It would be useful to be able to have a transaction that works across stores - probably a static method.
• a store.query instance method on a store without having to use the queryAll static method
• Tests of the query side, I have test of the materialise method.

This is really targeted at front end processing, rather than server side processing.

An Electric ShapeStream could easily be materialised to a Store.

[KyleAMathews]

Looks nice!

I think now that the basics are falling into place, it'll be easier to evaluate DX, perf w/ some e2e examples & real-world use cases we can implement and ship. I.e. this PR seems fine but it's harder to evaluate w/o the full context of its use.