Pair device dialog (#1217)

* WebUSB E2E experience

* Design feedback

* adding docs

* remove i got it

* bring back download again

* updated pair dialog wording

* tweak style

* make drive name strong

* fix isAvailable -> isEnabled

* upgrading pxt reference

* fixed typo

* bump pxt

pxtarget.json

@@ -90,7 +90,8 @@
                 "vid": "0x0d28",
                 "pid": "0x0204"
             }
-        ]
+        ],
+        "webUSB": true
     },
     "runtime": {
         "mathBlocks": true,