Squashed 'external/SDL/' content from commit 716c767b7

git-subtree-dir: external/SDL
git-subtree-split: 716c767b7e7099fb0e2fda9db59227a7e1d1d8a4

cmake/test/swift/module.modulemap

@@ -0,0 +1,4 @@
+module SDL3 [extern_c] {
+    header "shim.h"
+    export *
+}

cmake/test/swift/shim.h

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+/* Contributed by Piotr Usewicz (https://github.com/pusewicz) */
+
+#include <SDL3/SDL.h>