Keep context on "that" mark

[pokey]

Today, the "that" mark doesn't keep contextual information about its target. For example, if you refer to "arg", and then say "chuck that", it forgets that "that" was an argument, and thus doesn't clean up the trailing comma. We'd like to preserve the full target, including context, when we store the "that" mark

This functionality will be even more powerful when combined with #69, because it will enable chains to be broken up by stopping mid-way through a long chain, and using "that" as the mark to start the next section of the chain

See also #191; this functionality could be baked into the navigation map

Question: do we want to do the same for "this"?

• Add / update expansion tests (see also Re-evaluate decision to make non-empty selections non-weak #883)