feat(types): add types for $eval (#6135)

This pulls in the types (based on the DefinitelyTyped repo) for
`page.$eval` (and the `$eval` method on other classes). The `$eval`
method is quite hard to type due to the way we wrap and unwrap
ElementHandles that are passed to / returned from the `pageFunction`
that users provide.

Longer term we can improve the types by providing type overloads as
DefinitelyTyped does but I've deferred that for now (see the `TODO` in
the code for more details).

new-docs/puppeteer.elementhandle.aselement.md

@@ -7,9 +7,9 @@
 <b>Signature:</b>
 
 ```typescript
-asElement(): ElementHandle | null;
+asElement(): ElementHandle<ElementType> | null;
 ```
 <b>Returns:</b>
 
-[ElementHandle](./puppeteer.elementhandle.md) \| null
+[ElementHandle](./puppeteer.elementhandle.md)<!-- -->&lt;ElementType&gt; \| null