It only handles a few simple types right now, but the skeleton is there.

1. Still pretty janky.
2. Type Reference code doesn't work yet.
3. Other transforms just aren't done.
4. Always replaces, even when there isn't any transformation of JSDoc
types. (This probably isn't an issue since we wouldn't issue an error unless there were
some JSDoc to change.)

For ?, provide two code fixes, one for jsdoc/closure semantics
(`?t -> t | null)` and one for flow semantics
(`?t -> t | null | undefined`).
The current way of doing this is the hackiest thing you can imagine, but
it was easier than lifting everything into the list monad for a code fix
that I might not actually keep.

Instead of trying to walk the type structure in the codefix, I changed
to call getTypeFromTypeNode in the checker and then calling
typeToString. Together, these two functions normalise out JSDoc.
Note that you only get `T | null` for `?T` if you have --strict on. This
is technically correct -- adding a null union does nothing without
strict -- but it would still serve as documentation.