feat: workspace folders #3

[AntoineGS]

(Based off the same branch as the other two PRs, 372 and 373)
Adds support for workspaces, from the update README:

workspace_folders

Workspace folders improve Copilot's suggestions.
By default, the root_dir is used as a wokspace_folder.

Additional folders can be added through the configuration as such:

workspace_folders = {
  "/home/user/gits",
  "/home/user/projects",
}

They can also be added runtime, using the command :Copilot workspace add [folderpath] where [folderpath] is the workspace folder.

Fixes #375

[AntoineGS]

Upon further testing I seem to have introduced an issue with the attach/detach system, I will look into it!

[AntoineGS]

Ok I figured it out, explanation here, looking much better now!