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/main.ts

@@ -189,6 +189,7 @@ async function initCommonContext(context: vscode.ExtensionContext, ctx: Ctx) {
     ctx.registerCommand("resolveCodeAction", commands.resolveCodeAction);
     ctx.registerCommand("applyActionGroup", commands.applyActionGroup);
     ctx.registerCommand("gotoLocation", commands.gotoLocation);
+    ctx.registerCommand("cargoWorkspaceRootForCurrentRun", commands.getCargoWorkspaceDir);
 
     ctx.registerCommand("linkToCommand", commands.linkToCommand);
 }