feat: import postvariant.h if it exists

This commit adds a conditional include for "postvariant.h" in the
Arduino.h header file. If it exists, it will be included after the main
Arduino definitions. This allows for additional variant-specific
configurations or overrides to be applied without modifying the core
Arduino.h file directly.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>

Author: pillo79

cores/arduino/Arduino.h

@@ -160,4 +160,8 @@ void analogWriteResolution(int bits);
 // Allow namespace-less operations if Arduino.h is included
 using namespace arduino;
 
+#if __has_include("postvariant.h")
+#include "postvariant.h"
+#endif
+
 #endif // __cplusplus