Add more predefined variables

[PatOConnor43]

Summary

This PR adds more predefined variables like ${file} and ${workspaceFolder} which should match VS Code examples defined here.

Motivation

I was trying to create a Go Delve configuration that used ${fileDirname} based off these docs. I don't know if you have any interest in supporting all of these but they were simple enough to construct so I figured I'd include as many as I could.

With this variable I can now construct a config that looks like this to debug all tests within a package:

    {
      type = "go",
      name = "Debug package tests", -- configuration for debugging test files
      request = "launch",
      mode = "test",
      program = "${fileDirname}/...",
    },

Also, it looks like ${file} in this repo is relative to the workspace but ${file} in VS Code is absolute. I don't know if that was intentional or not, just an FYI. Thanks for taking the time to review 😄

[mfussenegger]

Looks good to me. Could you also update the docs in the dap.txt?

[PatOConnor43]

@mfussenegger I think this is ready to review again. Thanks!

[mfussenegger]

Thanks again 👍
Merged the PR