customEditor toggle and save/preview keybinds

[sverg1]

This PR aims to resolve #86504.

Tested on macOS High Sierra, Windows 10, and Ubuntu 18.04.3 LTS. Used README.md in test/ as a visual reference for keybinding and editor tests.

Toggle

By pressing CMD+E on Mac, or CTRL+E on Windows/Linux, the editor window opens a new tab in the same window that show either the default text editor or the custom Markdown preview editor, depending on the which editor was previously active editor. The toggle command does not get executed if in a context that does not have custom editors.

Save-to-Preview

By pressing CMD+Y on Mac, or CTRL+Y on Windows/Linux, the editor saves the changes—if any— of the text editor and switches to the custom editor. This command does not get executed if the context does not have custom editors. No changes are made if the custom editor is the currently active editor.

Caveats

• I chose CMD+Y instead of CMD+S because I had a difficult time getting the command to execute with CMD+S. Experimenting with all the KeybindingWeight values in my custom save command changed nothing, so the next best thing was for me to use a different key altogether.
• Using CTRL+E to toggle editors on Windows triggered to "Go to File..." command. Upon further inspection, CTRL+E is a secondary source-defined binding for that command but only for Windows (Mac and Linux had only the primary CTRL+P bound).

[msftclas]

All CLA requirements met.

[mjbvz]

I think the expected behavior would be to have a single toggle command that does the following:

• If a custom editor is focused, then replace it with the normal text editor
• If a text editor is opened, then replace it with a custom editor (if one exists)

Let's get that working for unedited files first and then can figure out how to handle saves (because custom editors can also be edited)

[sverg1]

@mjbvz Would CMD+E still be the right way to go, given possible conflicting commands? If I don't have to worry about the secondary "Go to File" shortcut for Windows, that's fine, since the command runs otherwise.
I will scrap the save command and modify the toggle to replace the window as opposed to opening a new tab.

[mjbvz]

Looking good. Let me know if you have any questions about my comments

[mjbvz]

Thanks! This will be in the first VS Code 1.42 insiders builds later this week