Attrs initialization without `@attr` decorator

Hi there 😄

After upgrading from v1.1.4 to v1.3.2, I noticed that my attributes were no longer shadowed by `initializeAttrs()`. I started digging and stumbled upon this piece of logic introduced in #191:

https://github.com/github/catalyst/blob/a8fb3ba59a4ac258d0fe64a50ecd3b6997f5f016/src/attr.ts#L39

It prevents `initializeAttrs()` from doing anything the second time around, thus skipping the manual initialization of attributes. Also, it seems like this change was one of the main things addressed by the above-mentioned PR.

So, what would be the correct way now to initialize the attributes without the `@attr` decorator?

Thanks!

---

**Example**

```html

class InfoMessage extends HTMLElement {
open = true

connectedCallback() {
initializeAttrs(this, ['open'])
}
}

controller(InfoMessage)

```

1. `controller(InfoMessage)` wraps original `connectedCallback()` and calls:

https://github.com/github/catalyst/blob/a8fb3ba59a4ac258d0fe64a50ecd3b6997f5f016/src/core.ts#L14

2. `InfoMessage` is marked as `initialized` (`attrs.ts`):

https://github.com/github/catalyst/blob/a8fb3ba59a4ac258d0fe64a50ecd3b6997f5f016/src/attr.ts#L40

3. The original `connectedCallback()` is executed, but `initializeAttrs()` would hit that early-return condition.
4. `this.open` is stuck in its default state (`true`) 😔

Start by reading src/attr.ts around the initialized guard and src/core.ts where controller wraps connectedCallback(). Reproduce the InfoMessage example with initializeAttrs(this, ['open']) and data-open="false". Done means manual initialization without @attr applies the attribute value instead of leaving open at true.

索引モデルが issue の本文から書いたものです。