* Add "Recommend" action to extension viewlet

Related to #13456

* Address PR requested changes

* Change functionality and UX

- Button goes from "Add to workspace recommendations" (clickable) to
  "Adding to workspace recommendations..." (unclickable) to
  "Added to workspace recommendations" (unclickable).
- It is only visible if the current extension is installed and there's
  at least one folder in the root that doesn't have it recommended yet.
  In other words, if it's already recommended in every folder of the root,
  it is not visible.
- In a single-root setup, it's immediately added to the recommendations.
- In a multi-root setup, a quick-pick is displayed with only the folders
  where it is not yet recommended available as options.
- In a multi-root setup, the button will go back to
  "Add to workspace recommendations" (clickable) instead of the
  unclickable state if there are still folders remaining where the
  current extension is not yet recommended.
- An error or success notification is displayed after the work is done.
- Configuration files are created if don't exist, modified otherwise.

* Don't show the button if ext is recommended in ANY folder

In the previous commit, the button would be displayed as long as there
was at least one folder in the workspace where the extension wasn't
recommended.

Now, the button will be displayed only if the current extension is not
recommended in any of the folders, as suggested in
https://github.com/Microsoft/vscode/pull/50419#issuecomment-398216711

* Improvements based on PR suggestions

- Lowercasing the extension ID before comparing
- Directly styling & labeling the button on the `run` command after work is done instead of calling `update`.
- Fix & delete unnecessary styles

* Expose feature as command instead of button

After a discussion with @ramya-rao-a and other members of the team,
it was decided that it wasn't reasonable to have a dedicated button for
a feature that's rarely used in such a visible place.

So instead, the action will be exposed as a command that will only be
available when there's an extension open in the editor area. In contrast
with the previous implementation, this has the added benefit of
allowing the user to use this action to recommend uninstalled
extensions as well.

* Simplify conditions for AddToRecommendations action

* Move comments so that they dont get erased when recommendations are added programatically

* Remove from unwanted before adding to recommendations

* Array improvements

* Friendly error msg

Win32 User Install

Improve Dependencies and Contributions UI in the Extension editor

Change custom Checkbox to expose an onChange Event instead of taking an onChange callback in options.
Otherwise it's impossible to reference the checkbox from inside the callback while constructing a Checkbox

Allow to customise background color of the editor pane visible on the left and right side of the centered editor layout

fixes #52434