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.unwrapelementhandle.md

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+<!-- Do not edit this file. It is automatically generated by API Documenter. -->
+
+[Home](./index.md) &gt; [puppeteer](./puppeteer.md) &gt; [UnwrapElementHandle](./puppeteer.unwrapelementhandle.md)
+
+## UnwrapElementHandle type
+
+Unwraps a DOM element out of an ElementHandle instance
+
+<b>Signature:</b>
+
+```typescript
+export declare type UnwrapElementHandle<X> = X extends ElementHandle<infer E> ? E : X;
+```