Allow `vscode.env.asExternalUri` to return valid results for workspace URIs

[joaomoreno]

Our current API vscode.env.asExternalUri currently does two things:

• For /^https?/ URIs, it translates remote URIs into URIs which can be handled locally
• For ${appScheme} URIs, it massages extension handling URIs

What if we augment that API to handle any URI scheme, namely URIs which represent workspaces? Here's how that could look like:

• On the desktop, the API would return a URI with the ${appScheme} protocol which would open the given workspace when invoked. So, asExternalUri('file://Users/joao/Desktop') would return something like vscode://open?uri=file%3A%2F%2FUsers%2Fjoao%2FDesktop.
• On an embedder web scenario like, the API would return the exact URL which would be open in the browser if invoked with vscode.open.

[joaomoreno]

Things to test:

• VS Code can now handle uris such as code-oss:workspace?file:///home/joao/Work/vscode. It will treat the query of said URI as the URI of the workspace to open. In this case it would open the workspace in file:///home/joao/Work/vscode.
• The vscode.env.asExternalUri API should throw if the provided URI can't be opened as a workspace. This matches the current API behavior which would throw whenver the scheme would be different from https? or the app protocol scheme

cc @eamodio