Break apart tmux-fastcopy and fastcopy binary

[abhinav]

Separate into two binaries:

• fastcopy would read from stdin and display a widget to select text; all options are passed as CLI arguments or env vars
• tmux-fastcopy handles the tmux integration: looking up tmux config, etc. and calling the fastcopy binary

This would be a first step towards integration elsewhere, e.g. kitty terminal overlays, possibly nvim overlays?