[Document Link Provider] Adding Visual Studio Live Share support

[lostintangent]

This PR adds support for Visual Studio Live Share to the document link provider, by ensuring that URI construction is done using the VS Code APIs which preserve the correct scheme. This is necessary because "guests" within a Live Share session see remote files using the vsls: scheme, whereas the the host (and non-Live Share users) sees files using the file: scheme. Therefore, this change preserves the existing behavior, while allowing REST Client users to click on < foo.js links within the context of a collaboration session 🚀 Specifically, this PR makes the following two changes:

1. Use Uri.toString() as opposed to Uri.fsPath, since the later strips the Uri's scheme, which removes the vsls: portion that is critical for Live Share.

2. Use Uri.parse() as opposed to Uri.file(), since the later always appends the file:// scheme to the parse Uri, as opposed to parsing the provided string, and respecting its scheme (if there is one)

VS Live Share + REST Client integration is already really great, since the "host" can share a locally running REST API, and the "guest" can then author requests against it (see this video). This PR extends that experience by making sure the document links work for all participants in a collaboration session.

// CC @Huachao

[Huachao]

Thanks @lostintangent ! It's really a great addition to REST Client extension.