pw_web_ui: Create mocks for testing WebSerial

Node and the browser have different stream APIs. This CL creates mocks for testing web_serial_transport, which uses the browser API.

Change-Id: Ice382bb89b10a2fa23fd4a39f9217e1d0c32a9cd
diff --git a/package.json b/package.json
index e5a1881..cdda168 100644
--- a/package.json
+++ b/package.json
@@ -9,6 +9,7 @@
     "@bazel/typescript": "^1.7.0",
     "@types/jasmine": "^3.5.10",
     "jasmine": "^3.5.0",
+    "rxjs": "^6.5.5",
     "typescript": "^3.9.3"
   }
 }