[rustdoc] Add new (cfg) feature search filtering/tab

[ralfbiedert]

When searching via s, it would be nice to find items affected by a certain feature, and to allow valid feature names to be searchable.

Take Tokio, which reports it supports the test-util feature:

When I now try to search for test-util, I get this:

Given that the docs are already somewhat aware of the used features per item

It would be nice if, for example, the search result tabs could have a category listing the affected items instead:

[GuillaumeGomez]

Moved it to the rust repository as you are asking about a rustdoc feature.