Add a command to enter a workspace without window reload

[bpasero]

This will help extensions like LiveShare that create ad-hoc workspaces to perform the switch from empty window to workspace without a window reload much faster.

Verify:

• write an extension (hello world is fine)
• craft any workspace file on disk
• add code like this: vscode.commands.executeCommand('_workbench.enterWorkspace', vscode.Uri.file('<path to workspace file'));
• you should enter this workspace without a window reload

[isidorn]

The new workspace seems to be opened without a reload, however the extension host is not started properly.
Follow your steps from above and wait 10 seconds and get an error -> reopening and assigning to August

[bpasero]

I know this used to work when I tested, so I speculate @sandy081 your changes might have caused this maybe after my changes were pushed?

[bpasero]

Very likely same symptoms as #55025, I see the extension host restarts but it is stopped in debug break.

[weinand]

Verified on macOS, Windows, and linux.