Add support for duck types with multiple methods to TypesExplainer

[dmolesUC]

Description

• Adds support for multiple-method duck types delimited with &, cf. @lsegal's suggestion in this StackOverflow comment from 13+ years ago.

Discussion

I agree with the sentiment expressed in that comment that doing this is not a great idea, and in general, it would be better to define an ad-hoc "interface" type. But there are times when you're dealing with an obnoxious third-party library, and you just need to document something in one place, and introducing a whole type for that feels like overkill.

This is a basic implementation that just covers this use case, but I could see an argument for instead supporting & for intersection types in general (and maybe | as well as , for union types while we're at it), which wouldn't be much more code.

Completed Tasks

• I have read the Contributing Guide.
• The pull request is complete (implemented / written).
• Git commits have been cleaned up (squash WIP / revert commits).
• I wrote tests and ran bundle exec rake locally (if code is attached to PR).