fix: in VSCode, correctly resolve relative paths to errors

VS Code problem matcher are restricted to be static "regexes". You can't
create a problem matcher dynamically, and you can't use custom code in
lieu of problem matcher.

This creates a problem for rust/cargo compiler errors. They use paths
relative to the root of the Cargo workspace, but VS Code doesn't
necessary know where that root is.

Luckily, there's a way out: our current problem matcher is defined like
this:

    "fileLocation": [ "autoDetect", "${workspaceRoot}" ],

That means that relative pahts would be resoleved relative to workspace
root. VS Code allows to specify a command inside `${}`. So we can plug
custom logic there to fetch Cargo's workspace root!

And that's exactly what this PR is doing!

editors/code/src/run.ts

@@ -89,12 +89,14 @@ export async function selectRunnable(
 
 export class RunnableQuickPick implements vscode.QuickPickItem {
     public label: string;
+    public cargoWorkspaceRoot?: string;
     public description?: string | undefined;
     public detail?: string | undefined;
     public picked?: boolean | undefined;
 
     constructor(public runnable: ra.Runnable) {
         this.label = runnable.label;
+        this.cargoWorkspaceRoot = runnable.args.workspaceRoot;
     }
 }