add multi-root file viewer

[lonhutt]

Added root level file only viewer when in multi-root mode.

Needed a nice way to open/interact with files stored in the project root when that folder was not rendered in the main explorer view.

[github-actions]

Test Results

 2 files  ±0   2 suites  ±0   1d 2h 0m 56s ⏱️ - 2h 56m 47s
 7 tests ±0   7 ✅ ±0  0 💤 ±0  0 ❌ ±0 
14 runs  ±0  14 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit bbd13b8. ± Comparison against base commit 4f12bc1.

♻️ This comment has been updated with latest results.

[guw]

@lonhutt Although it looks nice and useful I can see a conflict when you are working with multiple Bazel workspaces. Which one would it show?

Wouldn't it be better if there is a check in the background and a command for easily importing/adding a project for the workspace root into the multi-root setup? That seems more flexible than a second view.

Also I am seldomly using the views on the lower left. I really just focus on the file explorer and would expect the things be visible there.

[lonhutt]

@guw The idea here was to provide a simple way for users, when in a multi-root workspace, to view/open files like WORKSPACE, package.json, .buildifier.json, etc without having to include the full project root.

I think adding a 'import project' feature and improving the multi-root workspace management features is a really good idea, but not the purpose of this PR.

The alternative to this approach would be to create a separate folder in some other location (user.home?) and symlink in all the files stored at the root of the project.

As for the situation where multiple bazel projects are included in the workspace; do you mean the workspace would include only sub-modules from two or more bazel projects, or would you expect multiple bazel projects be included in their entirety, or do you expect submodules from one to be included alongside a complete/separate bazel project?