Allow using PROGMEM outside of Arduino (fixes #1903)

Ported from 40daf56b5a

extras/tests/Helpers/avr/pgmspace.h

@@ -0,0 +1,31 @@
+// ArduinoJson - https://arduinojson.org
+// Copyright © 2014-2023, Benoit BLANCHON
+// MIT License
+
+#pragma once
+
+#include <stdint.h>  // uint8_t
+
+#define PROGMEM
+
+class __FlashStringHelper;
+
+inline const void* convertPtrToFlash(const void* s) {
+  return reinterpret_cast<const char*>(s) + 42;
+}
+
+inline const void* convertFlashToPtr(const void* s) {
+  return reinterpret_cast<const char*>(s) - 42;
+}
+
+#define PSTR(X) reinterpret_cast<const char*>(convertPtrToFlash(X))
+#define F(X) reinterpret_cast<const __FlashStringHelper*>(PSTR(X))
+
+inline uint8_t pgm_read_byte(const void* p) {
+  return *reinterpret_cast<const uint8_t*>(convertFlashToPtr(p));
+}
+
+#define ARDUINOJSON_DEFINE_PROGMEM_ARRAY(type, name, ...)                \
+  static type const ARDUINOJSON_CONCAT2(name, _progmem)[] = __VA_ARGS__; \
+  static type const* name = reinterpret_cast<type const*>(               \
+      convertPtrToFlash(ARDUINOJSON_CONCAT2(name, _progmem)));