WebBLE partial flashing experiment. (#1388)

* support for webble

* more refactoring

* updated check

* flags

* moving flashing to experiments

* always have pairing mode

* fix settings

* added experiment image

* bump pxt

* add missing types

* bump pxt

pxtarget.json

@@ -275,7 +275,8 @@
             "allowPackageExtensions",
             "instructions",
             "debugger",
-            "bluetoothUartConsole"
+            "bluetoothUartConsole",
+            "bluetoothPartialFlashing"
         ],
         "bluetoothUartFilters": [{
             "namePrefix": "BBC micro:bit"