PSEditor should expose a list of open documents from the LSP

[JustinGrote]

Prerequisites

• I have written a descriptive issue title.
• I have searched all issues to ensure it has not already been requested.

Summary

PSEditor is an interface to PSES for the user. We expose the workspace and folders but we don't expose currently open documents from the LSP.

Proposed Design

The $PSEditor.Workspace variable is of type Microsoft.PowerShell.EditorServices.Extensions.EditorWorkspace.

This class should have a Documents property that enumerates all documents in the current workspace that are open in PSES. The documents should have Open and Save methods that map to the OpenFile and SaveFile methods of the EditorWorkspace.

Use commit 29617f8 as a reference.