Add support for shell autocompletion

[patrickfreed]

We should (optionally) install tab-completion scripts for swiftly. This will make it easier for users to remember certain commands and to select among installed toolchains.

[adam-fowler]

Should this have been closed? Not sure PR is linked to correct issue.

[patrickfreed]

Whoops, yes thanks for catching this. #69 was the one meant to be closed.

[cmcgee1024]

This is actually coming out of the box from the swift-argument-parser library that we use. Do we need to keep this open? Maybe this is a placeholder for writing some docs describing how to do it from swiftly?

https://apple.github.io/swift-argument-parser/documentation/argumentparser/installingcompletionscripts

[cmcgee1024]

#200 adds documentation for shell auto-completions with swiftly and should be relevant for the last swiftly release 0.3.0 too.

[cmcgee1024]

I'm closing this issue since shell autocomplete is there and now it is documented too: https://swiftpackageindex.com/swiftlang/swiftly/main/documentation/swiftlydocs/shell-autocompletion