[Discussion] [Live Share] Restricting language services to local files

[lostintangent]

In preparation for Visual Studio Live Share adding support for "guests" to receive remote language services for Ruby, this PR makes the following changes:

1. Updates the current DocumentSelector to be limited to file and untitled (unsaved) files.
2. Prevents the linter from running on workspaces that don't appear to have a root path (which a Live Share/virtual file system wouldn't). The task and intellisense providers already did this, so I made some minor refactorings to gate them all behind the same check.

With these changes, when someone has the Ruby extension installed, and joins a Live Share session (where files use the vsls: scheme), their language services will be entirely derived from the remote/host side, which provides a more accurate and project-wide experience (guests in Live Share don't have local file access to the project they're collaborating with).

If someone joins a Ruby project using Live Share, and doesn't have the Ruby extension installed, then they will automatically receive language services from the host (which is awesome! 🎉), so this PR is simply an optimization for the case where collaborating developers both have the Ruby extension installed. Additionally, this wouldn't impact the "local" Ruby development experience.

Note: As an example, the TypeScript/JavaScript language services that come in-box with VS Code already have this scheme restriction, and so this PR replicates that behavior.

[lostintangent]

@wingrunr21 The failing tests appear to be related to a debug adapter issue (unrelated to this PR), but that appears to have been resolved in master now.

[wingrunr21]

Awesome, thanks for this.

@gurgeous FYI on this feature set for your implementation

[lostintangent]

@wingrunr21 Thanks! Out of curiosity: do you have any plans to update the extension in the marketplace anytime soon?