Drop Flow support

The bug fix in this major version update is not obviously compatible with Flow, because Flow and TypeScript has different ways of treating classes. [Flow nominally types classes][0], which allows an empty class to constrain `NonEmptyArray`, whereas [TypeScript classes are compared structurally][1]. Supporting both represents a big tooling headache. Internal projects that use this library are all moving toward TypeScript; legacy projects using Flow should just remain on the v2 version, since using it with the Flow types should present no issue. If it turns out there is a need to support Flow and apply whatever updates v3 and beyond might get, the topic could be revisited.

[0]: https://flow.org/en/docs/lang/nominal-structural/#toc-classes-are-nominally-typed
[1]: https://www.typescriptlang.org/docs/handbook/2/classes.html#relationships-between-classes