Improve @DependencyEntry diagnostics

[stephencelis]

In the spirit of the checks we've added elsewhere, let's better surface a few error messages that are currently buried in the macro.

Default MainActor isolation

We currently allow this to compile in modules with default MainActor isolation:

extension DependencyValues {
  @DependencyEntry var client = Client()
}

This produces a non-sendable \.client key path because it's bound to the main actor, which fails only later when you try to use it:

@Dependency(\.client) var client  🛑

We can instead bury a default isolation check directly in the macro and surface it early:

Non-sendable dependencies

Dependencies must be sendable but currently if you try do:

extension DependencyValues {
  @DependencyEntry var nonSendable = NS()
}

The failure is buried in the macro expansion. Let's surface it instead:

Non-matching types

It's possible to invoke @DependencyEntry with mismatched types, especially if you forget to specify any Client as an existential. This produces several errors all buried in the expansion, but we can surface these as well: