ArrowFunctionExpression fails to check arrow functions without body and the type is also wrong

There are arrow functions whose body is a JSXExpression, like this:

```ts
const bla = (name: string) =>

That function will return false if you run `ArrowFunctionExpression.check(path)` and also the body type is wrongly defined here:
https://github.com/benjamn/ast-types/blob/b99f9b3ef5000631c06d71d91ee7aa341c9a81fb/gen/namedTypes.ts#L339

Because the type of that body is JSXElement, which doesn't have a nested body, which the other two does. This can lead to uncaught bugs.

Start in gen/namedTypes.ts around the ArrowFunctionExpression definition referenced by the issue, then inspect ArrowFunctionExpression.check(path) and the related arrow-function body types. Ensure JSXElement is represented correctly alongside the other supported body types, and verify that the resulting type check handles the JSX-bodied example without uncaught errors.

Written by the indexing model from the issue text.