This commit add Cargo-style project discovery for Buck and Bazel users.

This feature requires the user to add a command that generates a
`rust-project.json` from a set of files. Project discovery can be invoked
in two ways:

1. At extension activation time, which includes the generated
   `rust-project.json` as part of the linkedProjects argument in
    InitializeParams
2. Through a new command titled "Add current file to workspace", which
   makes use of a new, rust-analyzer specific LSP request that adds
   the workspace without erasing any existing workspaces.

I think that the command-running functionality _could_ merit being
placed into its own extension (and expose it via extension contribution
points), if only provide build-system idiomatic progress reporting and
status handling, but I haven't (yet) made an extension that does this.

editors/code/src/config.ts

@@ -214,6 +214,10 @@ export class Config {
         return this.get<boolean>("trace.extension");
     }
 
+    get discoverProjectCommand() {
+        return this.get<string[] | undefined>("discoverProjectCommand")
+    }
+
     get cargoRunner() {
         return this.get<string | undefined>("cargoRunner");
     }