Use a graph flag to control whether the graph should be used without resolution

[st0012]

We don't have a use case for this kind of usage yet, but if we want to support using rubydex without resolution, we want to let user explicitly configure that so the resolution items don't accumulate and waste memory.

Based on our discussion, the design should look like this:

graph = Graph.new(without_resolution: true) # or `resolution: false`?
# there will be no setter to change this
graph.index_source(...) # doesn't create pending work items

graph.resolve() # raise