Add 2 index arguments to custom shell completion calls

[rgoldberg]

Add 2 index arguments to custom shell completion calls.

They indicate to the Swift custom completion function:

1. the word for which completions are being requested.
2. the location of the cursor within that word.

Resolve #757

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[rgoldberg]

@natecook1000 This is the last of my completed PRs.

The next one would be for ToolInfoV0 (#758), but that can only be a draft because I think ToolInfoV0 must be updated for completions to be based on it.

I will look into #761 & try to resolve it soon.

Other issues remain with the completions, but all shells should be in a better state than before.

[natecook1000]

Looks great! I verified locally that the deprecation messages are coming through properly. 👍🏻

[rgoldberg]

@natecook1000 Pushed a spacing fix using Scripts/format.sh.

[natecook1000]

@swift-ci Please test