Break apart tmux-fastcopy and fastcopy binary

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?

The current codebase is a single Go binary. Start by examining the main.go and any tmux-specific integration code. Identify the separation point: the core text selection widget logic should go into a new 'fastcopy' binary, while tmux configuration handling remains in 'tmux-fastcopy'. Look for existing build scripts or Makefile to understand the build process. 'Done' means two separate binaries that work together as before, with tmux-fastcopy invoking fastcopy.

Written by the indexing model from the issue text.