Add `OS` module

js/modules/os.js

@@ -0,0 +1,10 @@
+/* eslint-env node */
+
+exports.isMacOS = () =>
+  process.platform === 'darwin';
+
+exports.isLinux = () =>
+  process.platform === 'linux';
+
+exports.isWindows = () =>
+  process.platform === 'win32';

preload.js

@@ -101,6 +101,7 @@
 
   // ES2015+ modules
   window.Signal = window.Signal || {};
+  window.Signal.OS = require('./js/modules/os');
   window.Signal.Types = window.Signal.Types || {};
   window.Signal.Types.Attachment = require('./js/modules/types/attachment');
   window.Signal.Types.Message = require('./js/modules/types/message');