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!

从 src/attr.ts 开始，重点查看 defineObservedAttributes() 和 getAttrNames()，然后将它们的行为与 docs/_guide/attrs.md 中的 vanilla 示例进行比较。确认手动初始化的属性是否可以在没有 @attr 的情况下被观察；当实现和指南对如何指定被观察的属性达成一致时，即表示完成。

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