Auto merge of #13367 - matklad:fix-problem-matchers, r=Veykril

Auto merge of #13367 - matklad:fix-problem-matchers, r=Veykril

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!