Add module selectors proposal (#2914)

* Add module selectors proposal

* Minor formatting and wording improvements

* Make changes based on parsing code review

* Clarify and expand discussions in pitch

Particularly future directions and alternatives considered.

* Flip-flop on restricting newlines after `::`

If reviewers don’t like it, we can revisit the question.

* Late edits to module selector proposal

* Describe keyword behavior more specifically
* Allow module selector in `@available(renamed:)`
* Use different Markdown syntax for links

* Module selectors is SE-0491

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Co-authored-by: Freddy Kellison-Linn <[email protected]>

Author: beccadax