feat: add floating terminal window option for flix commands

[Blackmorse]

Summary

My usual flow for running Flix commands is to pop them into a floating terminal, read the output, then close it — rather than keeping a persistent split around. Until now flix_cmd always opened a belowright split with no way to change that.

This PR makes the terminal behavior configurable via a new terminal option, so the split can be replaced with a floating window.

Changes

• terminal.window — "split" (default, keeps the existing belowright split) or "float" (opens a floating window).
• width / height — accept a fraction of the editor size (e.g. 0.5) or an exact column/row count (integer).
• pos_x / pos_y — accept a number or "center".
• close_binding — Normal-mode key that closes the floating window (default q).

Example

require("flix").setup({                                                                                                                                                                                                                        
  terminal = {                                                                                                                                                                                                                                 
    window = "float",                                                                                                                                                                                                                          
    width = 0.60,                                                                                                                                                                                                                              
    height = 0.5,                                                                                                                                                                                                                              
    pos_x = "center",                                                                                                                                                                                                                          
    pos_y = "center",                                                                                                                                                                                                                          
    close_binding = "q",                                                                                                                                                                                                                       
  },                                                                                                                                                                                                                                           
})