[Feature] Tabs for concurrent resource views

Is your feature request related to a problem? Please describe. Yes. k9s keeps a single navigation stack and a single active namespace. When I'm working through a problem in one namespace, filtered down to a few rows and drilled a couple of levels deep, and I need to check something in another namespace, I have to leave that view and rebuild all of it when I come back. Over a debugging session where I'm going back and forth between two or three namespaces or resource types, that turns into a lot of re-navigating.

Describe the solution you'd like Browser-style tabs. Ctrl-t opens a new tab, Ctrl-n / Ctrl-o cycle through them, and Alt-1 through Alt-9 jump straight to one. Each tab has its own navigation stack, filter, namespace, selection and scroll position, so moving between them is instant and nothing is lost.

Only one view is on screen at a time. These are tabs, not split panes, so shell, logs and everything else work exactly as they do now. The tab strip only appears once a second tab exists with one tab the UI is unchanged.

Describe alternatives you've considered

• [ / ] history keys. Good for retracing steps, but they re-run the command and rebuild the view, so the filter and scroll position I had are gone.
• Running several k9s instances side by side in tmux.

Additional context I've already built this and have been running it. Short demo:

It's fairly self-contained: the core (model.Stack, command.go, watch.Factory) is untouched, roughly 140 lines added plus new files with tests. Would you take a PR for this?