psram-aware malloc functions (backport of upstream #4895) (#342)

Introduces new memory allocation functions, based on wled#4895 by @DedeHai

* keep a minimum of 15KB RAM available to UI - improves stability
* S3/S2/C3 automatically use "fast RTC Ram" for small data (reduces fragmentation)
* auto-detects PSRAM (or no PSRAM) when firmware was built with -D BOARD_HAS_PSRAM
* d_malloc() and d_calloc() prefer DRAM if possible (faster), but fall back to PSRAM when free RAM gets low.

wled00/json.cpp

@@ -1133,6 +1133,8 @@ void serializeInfo(JsonObject root)
   #if defined(ARDUINO_ARCH_ESP32)
     root[F("freestack")] = uxTaskGetStackHighWaterMark(NULL); //WLEDMM
     root[F("minfreeheap")] = ESP.getMinFreeHeap();
+    auto maxFreeBlock = getContiguousFreeHeap();
+    root[F("maxalloc")] = maxFreeBlock;  // for upstream WLED compatibility
   #endif
   #if defined(ARDUINO_ARCH_ESP32)
   #if defined(BOARD_HAS_PSRAM) || (ESP_IDF_VERSION_MAJOR > 3) // V4 can auto-detect PSRAM