Outside of JSDoc comments, postfix-? is parsed at lower precedence than
the `?` of conditional types, and a postfix-? inside a tuple type
results in the type being marked optional.
This PR changes JSDoc parsing to behave the same way, which means that
1. Conditional types are allowed in JSDoc. Fixes#37166.
2. Tuple types' postfix-? syntax is interpreted correctly in JSDoc.
Fixes#38747.
The breaking change is that a postfix-? type followed by another postfix type,
like `[]` or `!`, is parsed as a conditional type. [Postfix-? is not
common](https://github.com/microsoft/TypeScript/issues/37166#issuecomment-612274456),
so this is an acceptable breaking change.
A postfix-? type `T?` is still parsed everywhere else and treated as `T | null`.
* Parse JSDoc ...T and T= only at top-level JSDoc
...T and T= should only be legal at the top level of a type, and only in
JSDoc, since at least T= is ambiguous elsewhere. This PR changes parsing
to make that happen. The resulting parse tree is now simpler, allowing
me to get rid of some code I had to add in the checker.
* Extract JSDoc type parsing into its own function
* PR comments:return from parseJSDocType
