Broadcast a "started" message after initialization

Other extensions listening in on NoScript's messages (e.g. Torbutton)
can take this message as an indication that it's now safe to send an
"updateSettings" message to NoScript without immediately getting
clobbered by the configuration loader.

See https://trac.torproject.org/projects/tor/ticket/26520 for context.

src/bg/main.js

@@ -180,6 +180,7 @@
           await include("/bg/Settings.js");
           Messages.addHandler(messageHandler);
 
+          await Messages.send("started");
           log("STARTED");
 
           this.devMode = (await browser.management.getSelf()).installType === "development";