Open workspace in ad-hoc (per-workspace) editor or IDE

[loujaybee]

Currently, if a user wants to use a different editor they are required to:

1. Go to preferences
2. Restart a workspace

Example use cases:

1. You want to use VS Code (web) editor for making "smaller" changes to documentation, markdown etc and having the in-browser functionality makes more sense.
2. You use a mono-repo with multiple languages or technology stacks and need or want an IDE for multiple aspects of that tech stack.
3. You have a mono-repo with multiple languages, but you also need the ability to have two editors or IDE's open for the same workspace for doing two different types of work simultaneously.

This issue is to add a method to directly open a workspace using X editor, overriding the general settings, allowing the user to choose an editor/IDE at workspace start time. This will also speed up the workflow for opening VS Code Desktop users, who currently have to go to the browser, but then use the command palette to open VS Code Desktop.

Relates to:

• Gitpod "open in" should be set-able per project, not just per-user #6874
• Open workspace with custom IDE via URL #9434
• Support start with other IDEs in Github browser-extension#59
• Create "Open In Gitpod (JetBrains)" button #7525
• [server] add editor prefix to choose custom IDE #9467

[loujaybee]

Closing as duplicate of: #6874