split up "normal completion" and "fuzzy completion integration"

- [x] I have read through the manual page (`man fzf`)
- [x] I have the latest version of fzf
- [x] I have searched through the existing issues

## Info

- OS
- [x] Linux (NixOS)
- [ ] Mac OS X
- [ ] Windows
- [ ] Etc.
- Shell
- [x] bash
- [ ] zsh
- [ ] fish

## Problem / Steps to reproduce
I think it would be good to split up the `completion.bash` file, and differentiate between the regular tab completion of options to fzf, which can be installed inside the normal bash completion directory like any completion script on the one hand, and the fuzzy completions of every other command, using fzf to display the possibilities on the other hand.

This enables users to opt for not using the fzf fuzzy completion for commands, but they can still get proper completion of fzf options. It also makes the packaging process easier, or at least more standardized.

I think the same thing also goes for zsh, but i haven't checked.

Start by reading completion.bash and the fzf manual's completion sections. Separate regular fzf option completion from fuzzy completion for other commands, so the former can be installed in the standard Bash completion directory while the latter remains optional. Check whether the same separation is needed for zsh, as suggested in the issue.

Written by the indexing model from the issue text.