Emit type declaration files when building library

[acelaya]

This PR introduces these changes:

1. Makes sure type declaration files are generated when building the library.
2. Defines the types entry point in package.json, so that consumers con discover those type declarations.
3. Adds a comment in the accessibility Scenario type, recommending returning a VNode when possible and explaining why.

Point 3 was the initial motivation for this PR, but then I realized that would not be available to consumers without publishing type declarations.

Our projects still don't use TS in tests, so the benefits this provides are limited at the moment, but I think it makes sense anyway as that's the direction we want to eventually go.

[robertknight]

Remind me: What are the unexpected results?

[acelaya]

Let's imagine this case:

function MyComponent() {
  return (
    <>
      <label htmlFor="the-input">Label<span>
      <input type="text" id="the-input" />
    </>
  );
}

checkAccessibility({
  content: () => mount(<MyComponent />),
});

Internally, when an enzyme wrapper is provided, we do wrapper.getDOMNode() and mount that in an actual DOM container.

In this particular case, that will make the test fail, because only the label will be mounted, as getDOMNode() can only return one node, and the fragment is not a valid one.

On the other hand, if the component is passed as is, we internally create the enzyme wrapper attaching the result to the DOM (mount(wrapper, { attachTo: container })), which results in both the label and the input being properly mounted.

checkAccessibility({
  content: () => <MyComponent />,
});

I didn't know how to synthesize this in a single sentence 😅

Also, I can see other cases where this could also happen, like when a component returns an array of children instead of a fragment:

function MyComponent() {
  return [
    <label key={1} htmlFor="the-input">Label<span>,
    <input key={2} type="text" id="the-input" />
  ];
}

[acelaya]

I have updated the comment to:

Returning a VNode is preferred. An Enzyme wrapper for a component with a
top-level Fragment will result in the test being run for the first DOM
child node only.
Use the Enzyme wrapper only if really needed, and consider wrapping your
component with a `<div />` in that case.