Adding serial.set(Rx|Tx)BufferSize (#1931)

* Fix for https://github.com/Microsoft/pxt-microbit/issues/1929

* bump pxt

* updated docs

* fix default values

* moving dialogs to react

* moving to react

* typo

* typo

sim/state/serial.ts

@@ -64,6 +64,14 @@ namespace pxsim.serial {
         // TODO
     }
 
+    export function setRxBufferSize(size: number) {
+        // TODO
+    }
+
+    export function setTxBufferSize(size: number) {
+        // TODO
+    }
+
     export function readBuffer(length: number) {
         if (length <= 0)
             length = 64;