chore(astro): Allow child elements in unstyled Astro components

Description

This PR adds the option to use custom elements/components inside our unstyled components, similar to the React counterpart, and adds a deprecation notice for the as prop.

With this change, we can now do:

---
import { SignInButton } from '@clerk/components/astro'
---
<SignInButton asChild>
  <button>Sign in with Clerk</button>
</SignInButton>

The new asChild prop determines if we should render the default slot as an element or just a string. We need this prop because Astro doesn't have a way to determine if a slot contains an element/component or a string.

Previously, we used the as prop to render a different element:

---
import { SignInButton } from '@clerk/components/astro'
---
<SignInButton as="button">
  Sign in with Clerk
</SignInButton>

The problem with the old approach was that we couldn't use an existing component or inherit styles. This PR offers better flexibility.

Resolves ECO-187

Checklist

[ ] npm test runs as expected.
[x] npm run build runs as expected.
[ ] (If applicable) JSDoc comments have been added or updated for any package exports
[ ] (If applicable) Documentation has been updated

Type of change

[ ] 🐛 Bug fix
[ ] 🌟 New feature
[ ] 🔨 Breaking change
[x] 📖 Refactoring / dependency upgrade / documentation
[ ] other:

clerk / javascript