js: Convert static/js/billing/billing.js to ES6 module.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

.eslintrc.json

@@ -139,7 +139,6 @@
                 "alert_words_ui": false,
                 "attachments_ui": false,
                 "avatar": false,
-                "billing": false,
                 "blueslip": false,
                 "bot_data": false,
                 "bridge": false,

static/js/billing/billing.js

@@ -1,6 +1,4 @@
-"use strict";
-
-exports.initialize = function () {
+export function initialize() {
     helpers.set_tab("billing");
 
     const stripe_key = $("#payment-method").data("key");
@@ -33,10 +31,8 @@ exports.initialize = function () {
         ]);
         e.preventDefault();
     });
-};
-
-window.billing = exports;
+}
 
 $(() => {
-    exports.initialize();
+    initialize();
 });

static/js/global.d.ts

@@ -13,7 +13,6 @@ declare let admin: any;
 declare let alert_words_ui: any;
 declare let attachments_ui: any;
 declare let avatar: any;
-declare let billing: any;
 declare let blueslip: any;
 declare let bot_data: any;
 declare let buddy_data: any;