Add tests for Json and Hex type

[Mrtenz]

Closes #40.

The test is almost completely copied from @metamask/types, with just one small adjustment. The following test would result in a compile-time type error:

// The Json type is too accepting of types. This type isn't valid Json because
// it can contain `undefined`, but the Json type allows it.
type TypeWithOptionalProperty = {
  a?: number;
};
const typeWithOptionalProperty: TypeWithOptionalProperty = { a: undefined };
expectAssignable<Json>(typeWithOptionalProperty);

TS2375: Type '{ a: undefined; }' is not assignable to type 'TypeWithOptionalProperty' with 'exactOptionalPropertyTypes: true'. Consider adding 'undefined' to the types of the target's properties.

I added undefined to a's type, which makes it not assignable to Json, which seems to be the behaviour we want.

[Mrtenz]

tsd seems to be built for declaration-only packages, as it looks at the types field in package.json (i.e., dist/index.d.ts, with no way to disable that behaviour it seems. For that reason, I moved it into a separate folder and added a dummy package.json.

[mcmire]

It sounds like, judging from the README, that if you pass a path to tsd, then it won't look in the types field. Is this not true in your experience? Do you think that people will try to run tsd on its own instead of yarn test:types?

[Mrtenz]

I wasn't able to get it to work using --typings or by passing a path, but apparently the behaviour is different if you specify a directory in the config. Seems to work fine now with the test being in the src folder.

[mcmire]

@Gudahtt Pinging you on this PR since you added these types originally!

[Mrtenz]

Simplified the tsd setup and moved the Hex type tests from the @metamask/types package as well, so now all the type tests are moved.

[mcmire]

This looks good to me, but I want to make sure Mark sees this.

[Gudahtt]

LGTM!

The test that was removed was rendered unnecessary by the exactOptionalPropertyTypes TypeScript option. An optional property can no longer be undefined.