Add resources DSL

[ZacSweers]

This streamlines configuration of enabling androidResources and enforces use of a resource prefix to avoid conflicts.

May your avatars never be wrongly sized again.

[github-actions]

Review:

To improve the code, you can make the following changes:

1. Add a null check for androidExtension in the resources() function to handle the case when it is null.
2. Move the null check for androidExtension outside the resources() function to avoid redundant checks.
3. Use the withType function to apply the changes to the LibraryExtension instead of casting and checking the type manually.

Here's the updated code:

public fun resources(prefix: String) {
  val libraryExtension = androidExtension as? LibraryExtension
    ?: error("slack.android.features.resources() is only applicable in libraries!")

  libraryExtension?.let { extension ->
    extension.withType<LibraryExtension> {
      resourcePrefix = prefix
      buildFeatures { androidResources = true }
    }
  }
}

By using the withType function, we can ensure that the changes are only applied to LibraryExtension and avoid casting and null checks.