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/ctx.ts

@@ -17,6 +17,10 @@ export type Workspace =
       };
 
 export class Ctx {
+    // Helps VS Code to correctly link problems from runnables. This is used by
+    // `rust-analyzer.cargoWorkspaceRootForCurrentRun` command of $rustc-run problem matcher.
+    cargoWorkspaceRootForCurrentRun?: string = undefined;
+
     private constructor(
         readonly config: Config,
         private readonly extCtx: vscode.ExtensionContext,