feat: add withConsolidated store helper

[manzt]

Towards #117

Introduces the withConsolidated store helper (?). Looks for v2 consolidated metadata,
and returns known contents. "Opening" paths from the store is essentially free for
consolidated data (and will not make a network request)

let store = await withConsolidated(new FetchStore("http://localhost:8080"));
let contents = store.contents() // [ {path: "/", kind: "group" }, { path: "/foo", kind: "array" }, ...]
let foo = await open(contents[1].path, { kind: "array" });