[IMP] add browser bindings to standard environment

This could be done by each application, but it does cost only a few
lines of code, and it helps standardizing the Owl ecosystem.

For example, some library (such as o_spreadsheet) needs to mock side
effects, and Odoo also needs to do that, so this prevents duplicated effort.

closes #686

src/browser.ts

@@ -0,0 +1,23 @@
+export interface Browser {
+  setTimeout: Window["setTimeout"];
+  clearTimeout: Window["clearTimeout"];
+  setInterval: Window["setInterval"];
+  clearInterval: Window["clearInterval"];
+  requestAnimationFrame: Window["requestAnimationFrame"];
+  random: Math["random"];
+  Date: typeof Date;
+  fetch: Window["fetch"];
+  localStorage: Window["localStorage"];
+}
+
+export const browser: Browser = {
+  setTimeout: window.setTimeout.bind(window),
+  clearTimeout: window.clearTimeout.bind(window),
+  setInterval: window.setInterval.bind(window),
+  clearInterval: window.clearInterval.bind(window),
+  requestAnimationFrame: window.requestAnimationFrame.bind(window),
+  random: Math.random,
+  Date: window.Date,
+  fetch: (window.fetch || (() => {})).bind(window),
+  localStorage: window.localStorage
+};