This patch fixes
rustwasm#2721. Let's consider
the following example:

```rust
pub struct Foo {
    /// Hello `first`.
    pub first: u32,

    /// Hello `second`.
    #[wasm_bindgen(readonly)]
    pub second: u32,
}
```

It outputs the following `.d.ts` file:

```typescript
export class Foo {
  free(): void;
/**
* Hello `first`.
*/
  first: number;
/**
* Hello `second`.
*/
  readonly second: void;
}
```

What's wrong here is that `second` has the type `void`. It should be
`number`.

What's happening? For `Foo.first`, a getter and a setter are
generated. The getter never has a return type (spoiler: that's the
bug), but the setter has a correct return type somehow (it's infererd
from its arguments, combined with a unit descriptor, see
`wasm_bindgen_cli_support::wit::Context::struct_`). When the getter
and the setter are processed, they both end up calling
`wasm_bindgen_cli_support::js::ExportedClass::push_accessor_ts`. This
function overwrites the return type for the same field everytime it is
called. So when called for the getter, the return type is missing, and
when called for the setter with a type, the bug is fixed.

For `Foo.second`, it's different. There is no setter generated because
the field is marked as `readonly`. So the bug appears.

To fix that, I've updated the getter `Function` descriptor, so that it
has an `inner_ret` value. This is passed to an `Adapter.inner_results`
at some point in the code, which is finally passed by
`JsFunction::process` to `JsFunction::typescript_signature` to
generate the typescript signature. And suddently, it's fixed!

However, we got:

```typescript
export class Foo {
  free(): void;
/**
* Hello `first`.
* @returns {number}
*/
  first: number;
/**
* Hello `second`.
* @returns {number}
*/
  readonly second: number;
}
```

We're making progress!

Nonetheless, the documentation is wrong now: The fields don't return
anything. This problem comes from the way `js_docs` is computed
globally for all export kinds in
`wasm_bindgen_cli_support::js::Context::generate_adapter`. This patch
also updates that to compute `js_docs` lately, for each branch. It is
specialized in the `AuxExportKind::Getter` and `AuxExportKind::Setter`
branches.

I hope this is the correct approach. I'm not sure about the difference
between `Adapter.results` and `Adapter.inner_results` as there is no
documentation unfortunately (kudos for every other super-well
documented place though).