Improved support for globs in multiroot workspace

[riccardoferretti]

Fixes #1063 #959

Replaces #1065

Currently globs are applied equally to all workspace folders
This approach is backwards compatible with single-root workspaces (and existing configuration)
It adds the possibility of limiting the glob to one of the directories by starting the glob pattern with the directory name.

Seems to be a bit of a complicated topic even within VS Code itself, see microsoft/vscode#82145 (comment) (amongst others)

How it works

In case there is more than one workspace folder, the glob can include the folder name to limit its application to that folder. If the glob is generic it still applies to all folders. For example, given:

notes/
  family/
    family.md
  business/
    business.md
  random/
    family-note.md

and

work/
  company/
    file.md
  side-project/
    project.md
  random/
    work-note.md

Given the random/** glob

1. if only the notes folder is open it will ignore everything inside the random directory (current behavior)
2. if both directories are open, it will ignore everything inside notes/random and work/random (current behavior)

If I only want to only ignore the random directory in notes, this was not possible.
With this PR, I can use the glob notes/random/** to target only one of the root directories.

Can it be confusing? Maybe but I am optimistic it won't:
when the workspace has a single folder, you don't see the folder in the explorer, so intuitively you wouldn't use it in your glob (which is also the behavior today, and the reason why on simple projects we haven't had issues).

When the workspace has more than a folder, then they show up in the explorer, so it does "feel" more right to include it in the glob

So the root folder "becomes" part of the explorer, and in the same way I can see how it could "become" part of the glob.

If a glob doesn't match a specific workspace folder it will apply equally to all of them (as it is today).