Attrs are never observed unless added with `@attr` decorator

Hi folks,

While I was testing the attributes — trying to work my way around #207 — I found another subtle detail. Manually initialized attributes are never observed.

Catalyst's guide provides the following example for vanilla users: https://github.com/github/catalyst/blob/main/docs/_guide/attrs.md#what-about-without-decorators.

Please notice the second argument there `defineObservedAttributes(HelloWorldElement, ['foo'])` and compare it with the actual function declaration:

https://github.com/github/catalyst/blob/a8fb3ba59a4ac258d0fe64a50ecd3b6997f5f016/src/attr.ts#L99

It doesn't expect the second argument (`arguments` aren't used either).

Now, internally `defineObservedAttributes()` pulls attribute names from `getAttrNames()`, which, in turn, pulls them from `attrs` (`WeakMap`). The only way to populate `attrs` is to use `@attr` decorator:

https://github.com/github/catalyst/blob/a8fb3ba59a4ac258d0fe64a50ecd3b6997f5f016/src/attr.ts#L15

Maybe I'm missing something, but it seems like there's no way to specify observed attributes without decorators. In any case, it'd be very nice to clarify that 🙂

Thank you!