feat: change QueryHandler to contain QueryOne and QueryAll methods

[johanbay]

Extends #5753 with support for registering both $ and $$ handlers under the same selection prefix, i.e myCustomQueryHandlerPrefix/<selector>.

[paullewis]

Generally LGTM, but I'll await Mathias' approval.

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[hanselfmu]

LGTM, thanks Johan!

[jackfranklin]

nice work getting rid of this as :)

[jackfranklin]

nice tests!

[jackfranklin]

This LGTM with one question about the change of type to Function and if we can avoid it.

[johanbay]

Summary of changes:
The experimental API for registering custom query handlers is slightly changed such that one registers query handlers containing two strategies: one for querying a single element and one for querying all matching elements.

The api for custom query handlers is unchanged:

__experimental_registerCustomQueryHandler(name: string, queryHandler: QueryHandler): void;
__experimental_unregisterCustomQueryHandler(name: string): void;
__experimental_customQueryHandlers(): Map<string, QueryHandler>;
__experimental_clearQueryHandlers(): void;

but QueryHandler is now defined as

export interface QueryHandler {
  queryOne?: (element: Element | Document, selector: string) => Element | null;
  queryAll?: (element: Element | Document, selector: string) => Element[] | NodeListOf<Element>;
}

[mathiasbynens]

Discussed with Paul offline. He's fine with this, so not blocking on his review+ bit.