Add GetMonitorScale() for PLATFORM_WEB

src/platforms/rcore_web.c

@@ -754,8 +754,11 @@ int GetMonitorRefreshRate(int monitor)
 // Get specified monitor scale
 Vector2 GetMonitorScale(int monitor)
 {
-    TRACELOG(LOG_WARNING, "GetMonitorScale() not implemented on target platform");
-    return (Vector2){ 1.0f, 1.0f };
+    // NOTE: Returned scale is relative to the current monitor where the browser window is located
+    Vector2 scale = { 1.0f, 1.0f };
+    scale.x = (float)EM_ASM_DOUBLE( { return window.devicePixelRatio; } );
+    scale.y = scale.x;
+    return scale;
 }
 
 // Get the human-readable, UTF-8 encoded name of the selected monitor