Experiment BT support using Chrome web serial (#920)

* plumbing

* plumbing

* logging

* more notes

* fixing typing

* more plumbing

* more plumbing

* different baud rate

* talking to the brick

* first over the air drop

* fix buffer

* tweak paraetmers

* formatting fixing double upload

* reduce console.log

* cleanup

* add BLE button to download dialog

* changed label

* recover from broken COM port

* fix function call

* reduce log level

* adding ticks

* some help

* updated support matrix

* more docs

* updated browser help

* more docs

* add link

* add device

* added image

docs/bluetooth.md

@@ -0,0 +1,51 @@
+# Bluetooth
+
+This page describes the procedure to download MakeCode program to the EV3 brick 
+over Bluetooth.
+
+## ~ hint
+
+### WARNING: EXPERIMENTAL FEATURES AHEAD! 
+
+Support for Bluetooth download relies on [Web Serial](https://wicg.github.io/serial/),
+an experimental browser feature. Web Serial is a work [in progress](https://www.chromestatus.com/feature/6577673212002304); 
+it may change or be removed in future versions without notice.
+
+By enabling these experimental browser features, you could lose browser data or compromise your device security 
+or privacy.
+
+## ~
+
+## Supported browsers
+
+* Chrome desktop, version 77 and higher, Windows 10
+* [Edge Insider desktop](https://www.microsoftedgeinsider.com), version 77 and higher, Windows 10
+
+To make sure your browser is up to date, go to the '...' menu, click "Help" then "About".
+
+## Machine Setup
+
+* pair your EV3 brick with your computer over Bluetooth. This is the usual pairing procedure.
+* go to [chrome://flags/#enable-experimental-web-platform-features](chrome://flags/#enable-experimental-web-platform-features) and **enable** 
+**Experimental Web Platform features**
+
+![A screenshot of the flags page in chrome](/static/bluetooth/experimental.png)
+
+## Download over Bluetooth
+
+* go to the **beta** editor https://makecode.mindstorms.com/beta
+* click on **Download** to start a file download as usual
+* on the download dialog, you should see a **Bluetooth** button. Click on the
+**Bluetooth** button to enable the mode.
+* **make sure the EV3 brick is not running a program**
+* click on **Download** again to download over bluetooth.
+
+## Choosing the correct serial port
+
+Unforunately, the browser dialog does not make it easy to select which serial port is the brick.
+On Windows, it typically reads "Standard Serial over Bluetooth" and you may 
+have multiple of those if you've paired different bricks.
+
+## Feedback
+
+Please send us your feedback through https://forum.makecode.com.