[ios] backport from 4.3-beta: expose explict module for c & cpp

- standard user can import spine-c based api as usual
- dedicated swift/cpp user can import cpp interface with explict import and enabling cpp interp
- objective-c user can import cpp using objective-c++

spine-ios/Sources/SpineCppLite/include/module.modulemap

@@ -1,4 +1,13 @@
 module SpineCppLite {
-    header "spine-cpp-lite.h"
-    export *
+    use c
+    export c
+    explicit module cpp {
+        umbrella header "spine/spine.h"
+        requires cplusplus11
+        export *
+    }
+    explicit module c {
+        header "spine-cpp-lite.h"
+        export *
+    }
 }