Add proposal to clarify `nullable` (#2050)

* Addressing #1389, and clearing the way for PRs #2046 and #1977. This adds a formal proposal to clarify the semantics of nullable, providing the necessary background and links to related resources.

* Corrected table formatting.

* Minor tweaks and corrections.

* Correct Change Log heading.

* Cleaned up notes about translation to JSON Schema and *Of inheritance semantics.

* Fix Change Log heading in the proposal template.

* Snappy answers to stupid questions.

* Change single-quote 'null' to double-quote "null"

Thanks, @handrews for the review.

* Clarified the proposed definition of nullable

Somehow in our collaborative editing, we neglected to state that `nullable` adds `"null"` to the set of allowed types. We just said that it "expands" the `type` value, but didn't state the obvious (to us) manner of said expansion. Correcting that oversight in this commit.

* Corrected the alternative, heavy-handed interpretation of nullable.

* Added more explicit detail about the primary use case.

* Added a more complete explanation of the problems created by disallowing nulls by default.