Convert usermods from header to library

usermods/sht/library.json

@@ -0,0 +1,9 @@
+{
+  "name:": "sht",
+  "build": {
+    "srcDir": ".",
+    "includeDir": "../../wled00",
+    "libLDFMode": "chain+",
+    "libArchive": false
+  }
+}

usermods/sht/sht.cpp

@@ -477,4 +477,7 @@ float ShtUsermod::getTemperature() {
  */
 const char* ShtUsermod::getUnitString() {
   return unitOfTemp ? "°F" : "°C";
-}
+}
+
+static ShtUsermod sht;
+REGISTER_USERMOD(sht);