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/commands.ts

@@ -842,6 +842,7 @@ export function run(ctx: Ctx): Cmd {
         item.detail = "rerun";
         prevRunnable = item;
         const task = await createTask(item.runnable, ctx.config);
+        ctx.cargoWorkspaceRootForCurrentRun = item.cargoWorkspaceRoot;
         return await vscode.tasks.executeTask(task);
     };
 }
@@ -946,3 +947,6 @@ export function linkToCommand(ctx: Ctx): Cmd {
         }
     };
 }
+export function getCargoWorkspaceDir(ctx: Ctx): Cmd {
+    return async () => ctx.cargoWorkspaceRootForCurrentRun;
+}