Fix WebUSB serial (#55)

Reinstate serial after flashing to fix starting of serial after flashing (main issue and related issue 1)
Don't start serial when flashing to fix flashing issues (related issue 2)

Fixes microbit-foundation/python-editor-v3#1209

Author: microbit-grace

lib/usb.ts

@@ -483,7 +483,12 @@ class MicrobitWebUSBConnectionImpl
   protected eventActivated(type: string): void {
     switch (type as keyof SerialConnectionEventMap) {
       case "serialdata": {
-        this.startSerialInternal();
+        // Prevent starting serial when flashing.
+        if (!this.flashing) {
+          this.startSerialInternal();
+        }
+        // Allows for reinstating serial after flashing.
+        this.addedListeners.serialdata++;
         break;
       }
     }
@@ -493,6 +498,7 @@ class MicrobitWebUSBConnectionImpl
     switch (type as keyof SerialConnectionEventMap) {
       case "serialdata": {
         this.stopSerialInternal();
+        this.addedListeners.serialdata--;
         break;
       }
     }