chore: Move dpi.rs to winit-core wholesale

Signed-off-by: John Nunley <dev@notgull.net>

winit-core/Cargo.toml

@@ -16,3 +16,7 @@ rust-version = "1.70.0"
 default = ["std"]
 std = ["alloc"]
 alloc = []
+serde = ["dep:serde"]
+
+[dependencies]
+serde = { workspace = true, optional = true }

winit-core/src/lib.rs

@@ -10,4 +10,5 @@
 #[cfg(any(not(feature = "std"), not(feature = "alloc")))]
 compile_error! { "no-std and no-alloc usage are not yet supported" }
 
+pub mod dpi;
 pub mod error;