Add Bash and Zsh command-line completion for flutter (#19243)

This uses @kevmoo's completion package to do command line completion for flutter, and a new command "bash-completion" (with alias "zsh-completion") that will output the necessary shell script setup code, and adds the hidden command "completion" that does the actual completion.

Because it adds a dependency, I also had to do flutter update-packages --force-upgrade.

Fixes #18988.