Settings editor should render objects with boolean values a list of checkboxes

[rzhao271]

For settings of type object where all the keys are boolean keys, we should render those settings as a list of checkboxes.

A setting to check this issue with is gitlens.advanced.messages.
Expected behaviour: the settings editor renders that setting as a list of checkboxes.
Actual behaviour: the settings editor does not render that setting. The user must edit that setting in a JSON file.

Code pointer: https://github.com/microsoft/vscode/blob/main/src/vs/workbench/contrib/preferences/browser/settingsTreeModels.ts#L611

CC @roblourens