Add plugin capabilities

[imsnif]

We should add more plugin capabilities to make them more powerful and allow the community's creativity to flow. Plugins should be able to:

• Read and change the scroll-buffer of each terminal pane
• Display graphics in the terminal (to be translated to a Sixel image, w3m frame-buffer dump, chafa-like image or whatever the user's terminal supports)
• Control the pane layout
• Read / switch between Zellij sessions

Depends on: #1395

[zegabr]

Looking forward to see the switch between sessions feature so I can start using it the same way I use tmux rn

[hhoeflin]

The missing switching of sessions is the last thing that keeps me from switching from tmux to zellij. And a session browser like tmux offers would be nice. Very impressive work on zellij - especially like the Sixel support.

[jade-tea-h]

I saw an issue where there was a discussion on security concerns was raised with permissions and allowing plugins to log keys, would it be possible to make one of the additional capabilities that plugins can add actions to zellij? That way the zellij side can handle when plugins act without requiring that the plugin be focused. I think this would make it easier to solve issues like #967 as well.