This implements the rudiments of completions within Rust.

What this can do for Bash

• Nest into subcommands
• Provide single-value positional value completions
  • PossibleValues
  • Some of ValueHint
• Provide short and long flag completions
• Provide subcommand completions

What this does not do

• Flag values
• Multiple values per argument
• Removing options based on conflicts
• Handle all of the random App and Arg settings
• Correctly append spaces when needed
• Shells other than Bash

Part of #3166