Move platform to common and remove dom dependence in common

Because common/ imports 'xterm', it also imported dom accidentally.

Fixes #1990

src/AccessibilityManager.ts

@@ -5,7 +5,7 @@
 
 import * as Strings from './Strings';
 import { ITerminal, IBuffer } from './Types';
-import { isMac } from './core/Platform';
+import { isMac } from './common/Platform';
 import { RenderDebouncer } from './ui/RenderDebouncer';
 import { addDisposableDomListener } from './ui/Lifecycle';
 import { Disposable } from './common/Lifecycle';