Check if /_vscode web app exists before redirecting .vscode files

[isc-bsaviano]

This PR reduces the number of unnecessary for REST requests for server-side folders that cannot redirect .vscode files. To do this, I check the list of %SYS web apps to see if /_vscode is in it. I verified that VS Code behaved correct when the /_vscode web app was enabled or disabled. While working on this change, I discovered a few other issues that I've fixed:

• Cache the list of web apps and abstract document types for all server-namespaces in the workspace.
• Combined our onDidChangeWorkspaceFolders event handlers into a single function.
• Always use FileSystemError when throwing errors in FileSystemProvider.

[gjsjohnmurray]

Slightly uneasy about having our FSP throw errors that aren't vscode.FileSystemError.* ones, but I see we already started doing this recently in another PR, so I'm going to approve this.

[isc-bsaviano]

@gjsjohnmurray I can roll that change back if you think that it's more important to always return a vscode.FileSystemError. It has a constructor so I can create errors that are not one of their predefined types. That will be useful for when their few defaults don't fit.

@isc-rsingh What do you think?

[gjsjohnmurray]

Doc for FileSystemError says (my added emphasis):

A type that filesystem providers should use to signal errors.

I'd be inclined to play safe and toe the line here.