Discover unlinked projects and suggest adding them to the settings

In rust-analyzer, it's important to know the set of opened projects up-front. This set of project is stored in the linkedProjects config.

It is possible to link projects dynamically & lazily -- the first time you open an rs file, find the corresonding toml and add it to the project. This has surprising interaction with find usages: if A uses B, and you open B and run fiend usages (or some refactor based on fiend usages), then you'd see usages in A only if you happened to open A in this editing session as well. So, we really want to know, up front, that both A and B exist.

What we could do though, is to notice a project which isn't link, and show a dialog in the edditor asking the user to (automatically), add the newly disccovered project to settings.json.

This needs some work on the server (to notice unlinked projects) and on the client (to actually modilfy the settings).

Read the server-side project discovery and client-side settings handling, focusing on linkedProjects and settings.json. Trace how an unlinked project is detected and how the editor can offer adding it; done means the server notices it and accepting the dialog updates settings so the project is linked up front. No specific files or tests are named, so repository tracing is required.