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

libs/core/pxt.json

@@ -72,7 +72,7 @@
                     "microbit-dal": {
                         "bluetooth": {
                             "open": 1,
-                            "pairing_mode": 0,
+                            "pairing_mode": null,
                             "whitelist": 0,
                             "security_level": null
                         }
@@ -98,7 +98,7 @@
                     "microbit-dal": {
                         "bluetooth": {
                             "open": 0,
-                            "pairing_mode": 1,
+                            "pairing_mode": null,
                             "whitelist": 1,
                             "security_level": "SECURITY_MODE_ENCRYPTION_WITH_MITM"
                         }