Compiler doesn't warn when using Ash.Policy.Authorizer in wrong place.

Sometimes when sleep deprived / not paying attention you may add `Ash.Policy.Authorizer` to the `extensions` list on your resource declaration (instead of in the `authorizers` list), you will then proceed to be flummoxed for the next few hours as your policies don't work - yet your resource has the policy DSL available in it.

Ideally this would throw an error.

Start at the resource declaration handling for the `extensions` and `authorizers` lists, focusing on how `Ash.Policy.Authorizer` is recognized. Reproduce the misplaced extension configuration and trace the compiler validation path; done means it reports an error when `Ash.Policy.Authorizer` is placed in `extensions` instead of `authorizers`.

Written by the indexing model from the issue text.