Add GettingStartedService/Registry and initial getting started UI

[JacksonKearl]

Ref #106715 / #106717.

UI is still work in progress. Enabled by flag workbench.experimental.gettingStarted: true.

CC @sandy081 / @bpasero for changes to contrib files. (workspaceActions, extensions, themes, quickInput)

[sandy081]

Approving for changes in extensions actions file

[bpasero]

@JacksonKearl instead of having to report the progress on tasks in each of the actions, why not listen to command execution in one central place via ICommandService.onDidExecuteCommand and then track progress through that? Maybe we can change the API that you register a task and the associated command IDs that will complete the task and the registry could listen and update progress accordingly?

[JacksonKearl]

@bpasero good idea, I've reverted most of the changes to external modules in favor of listening to onDidExecuteCommand, except for theme picker where I still explicitly emit an event when the user has selected a theme as opposed to simply running the command.