[Enhancement Proposal] Merge "Manage WorkTrees" and "Create WorkTree" dialogs into one

[ar-schraml]

First of all, thanks for this awesome extension, it's very helpful!

Proposal

I'd like to propose to merge the existing Manage WorkTrees and Create WorkTree dialogs into one.

Reason

I often want to switch to a worktree for a certain branch, but don't actually know if I already have a worktree for that branch. As the following to scenarios show, there isn't an optimal solution for that problem right now:

1. If I open the Manage WorkTrees dialog first and see that no worktree exists for the branch, I have to close the dialog and open the Create WorkTree dialog instead.
2. Conversely, if I open the Create WorkTree dialog first to try and create a new worktree for the branch only to be told that that a worktree for that branch already exists, I have to close the dialog and open the Manage WorkTrees dialog instead.

So in both cases, I have to do some extra clicks to close the current dialog and open the other one.

Now of course that's not a huge issue, but I think combining the dialogs into one could make the usage easier and more comfortable.