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/lsp_ext.ts

@@ -43,6 +43,10 @@ export const relatedTests = new lc.RequestType<lc.TextDocumentPositionParams, Te
     "rust-analyzer/relatedTests"
 );
 export const reloadWorkspace = new lc.RequestType0<null, void>("rust-analyzer/reloadWorkspace");
+export const addProject = new lc.RequestType<AddProjectParams, string, void>(
+    "rust-analyzer/addProject"
+)
+
 export const runFlycheck = new lc.NotificationType<{
     textDocument: lc.TextDocumentIdentifier | null;
 }>("rust-analyzer/runFlycheck");
@@ -68,6 +72,8 @@ export const viewItemTree = new lc.RequestType<ViewItemTreeParams, string, void>
 
 export type AnalyzerStatusParams = { textDocument?: lc.TextDocumentIdentifier };
 
+export type AddProjectParams = { project: JsonProject[] };
+
 export type ExpandMacroParams = {
     textDocument: lc.TextDocumentIdentifier;
     position: lc.Position;