add dfplayer mini extension (#2417)

docs/extensions.md

@@ -478,6 +478,10 @@ Check out [microbit.org](http://microbit.org/resellers/) for more information on
   "name": "51bit ColorBit",
   "url":"/pkg/51bit/ColorBit",
   "cardType": "package"
+}, {
+  "name": "51bit DFPlayer mini",
+  "url":"/pkg/51bit/dfplayermini",
+  "cardType": "package"
 }, {
   "name": "Kitronik Zip Tile",
   "url":"/pkg/KitronikLtd/pxt-kitronik-zip-tile",

targetconfig.json

@@ -140,6 +140,8 @@
             "sparkfun/pxt-gator-soil",
             "sparkfun/pxt-gator-microphone",
             "rebeccaclavier/pxt-bmp280",
+            "makecode-extensions/HTS221",
+            "51bit/dfplayermini",
             "makecode-extensions/STTS751",
             "makecode-extensions/LSM6DSO",
             "makecode-extensions/LPS22",