feat!: Separate `@arcjet/headers` package from core

[blaine-arcjet]

Closes #224

This separates the ArcjetHeaders package into its own package. This means it is no longer exported from arcjet and anyone consuming it will need to depend on @arcjet/headers directly.

Separating it out gave me a chance to think through unnatural usage in non-TypeScript environments so I added tests and guards around those situations. Having this package separate will allow us to improve/fix it independent of arcjet core.