Change swingset-runner modularity so we can move chain stuff out of it

[FUDCo]

What is the Problem Being Solved?

As of PR #8239 swingset-runner has acquired code to support emulating our chain setup. While this works fine, it embeds some chain-specific code into an application where it really doesn't belong from a modularity perspective. We'd like the swingset-runner code to be agnostic about chain-specific details, both so that it can be built and run without any of the chain stuff being present when the chain is irrelevant, and to accommodate hypothetical future applications that interact with the SwingSet from the outside in ways different from how our chain does it.

Description of the Design

This issue exists primarily so we don't lose track of the problem. There is no specific design for this right now, aside from a vague notion of adding a plugin scheme of some kind. The first part of addressing this issue will be to produce such a design.

A complete implementation should include actually fixing the modularity concern by moving the chain-specific portions of swingset-runner into a different package that makes use of whatever we come up with.