webrepl_cli.py: Update "get file" protocol.

"Get" protocol now requires receiving a lead byte from client before
transferring each new packet with file contents. This makes it naturally
non-blocking (while webrepl waits to receive this byte, MicroPython
can execute other tasks).

Author: Paul Sokolovsky

webrepl_cli.py

@@ -130,6 +130,7 @@ def get_file(ws, local_file, remote_file):
     with open(local_file, "wb") as f:
         cnt = 0
         while True:
+            ws.write(b"\0")
             (sz,) = struct.unpack("<H", ws.read(2))
             if sz == 0:
                 break