Custom type rendering support

My library whippyunits (and upcoming linear algebra adapter whippyalgebra) makes extensive use of deep, not very human readable generics. To deal with this, I ship a LSP proxy that wraps the rust-analyzer binary, intercepting the JSON messages, sniffing out my types, and rewriting them into human readable form:

This works exceptionally well - it is hard to overstate how helpful it is to be able to look at the shape and semantics of a matrix on mouseover - but it is not composable and it requires an annoying installation for my users.

A proper r-a feature that allows this kind of text manipulation would be extremely useful. Even better if it allows customization of autofill behavior on double-click (my LSP proxy replaces verbose Quantity types with simple qty! macro invocations). There's no need to support particularly deep reflection/metaprogramming in the text transformation layer - you get very good results even if you just parse the literal string of the type that would be rendered, and re-render it.

Start with the rust-analyzer binary and the JSON messages intercepted by the existing LSP proxy. Determine which rendered-type and double-click autofill behaviors would need a customization interface, then define a minimal composable design and verify it against the whippyunits examples described in the issue.