Add fig completion support

[migueldeicaza]

Fig published a scheme for commands and completion, and we could have swiftterm or SwiftTermApp support this:

https://github.com/withfig/autocomplete

Issues to track:

• installing shell variables to flag start/end of command execution
• how to make UITextInput offer our custom completions (this could steer the support to swift term), or if we should offer the completions as a separate UI (implementation would be in SwiftTermApp)
• loading, parsing a fig database of completions.
• UI could be done with this: https://github.com/IMcD23/InputAssistant