Improve the ergonomics of "use"

[philrz]

A couple examples have been recently cited where the current behaviors of "use" with a Zed lake may not match well with user expectations/needs. Below is a current list, with more to be added as we encounter them.

1. @mccanne has noted that instead of the current single/global cached "use" settings in /.zed_head, he sometimes wishes he could have different open shells with different cached "use" settings, e.g., perhaps with settings attached to different working directories.

2. Issue "zed compact" relies on "use" being set to the compacted pool #3971

Improvements in this area may be something we want to pursue at the same time we're making decisions about #3956.