feat(picker): add tmux pickers

[Doekeb]

Description

This PR adds sources for tmux entities.

• Sessions (preview shows selected session windows)
• Windows (preview shows selected window panes)
• Panes (preview shows a preview of the selected pane)
• Clients (preview shows a preview of the client's focused pane)
• Tree
  • Shows the session > window > pane hierarchy
  • Preview shows the preview for the selected entity

The default select for all of the sources is to focus the selected entity (for a client, focus the client's currently active pane).

Of course, users can define and keymap whatever other actions they wish. Some possible examples (not included in this PR):

• tmux/nvim interactions (more interesting)
  • yank the visible contents of the selected pane to a register
  • send the contents of a register to the selected pane
• pure tmux operations (less interesting)
  • kill the selected pane/window/session
  • detach the selected client

The motivating example which I use for my own workflow (also not included in this PR) is to select a tmux pane as a target for slime:

opts = {
  picker = {
    actions = {
      slime_select = function(picker, item)
        if item.pane_id then
          picker:close() -- Close the picker first so buffer options apply to the buffer we opened the picker from
          vim.g.slime_default_config = { socket_name = "default", target_pane = item.pane_id }
          vim.b.slime_config = { socket_name = "default", target_pane = item.pane_id }
        end
      end,
    },
  },
}

Screenshots

Panes:

Windows:

Sessions:

Clients:

Tree (Sessions > Windows > Panes):