feat: Add filter for current session only

[dimtion]

This is a small PR to add an option to provide suggestions only for the tmux pane of the current session nvim is in.

I use sessions a lot for different projects, and so mixing sessions causes a lot of false positives.

Let me know if I need any iteration on the code.

[mgalliou]

Hi, great suggestion.

However, with your changes it's possible to have both all_panes and session_panes enabled, which doesn't make much sense. Maybe we could introduce a panes field replacing the all_panes field with possible values being all, session, and window (window would be the default).

This would introduce a breaking change, but i think this is fine to do so since it doesn't change the default behaviour.

Let me know what you think @dimtion 😉