Move platform to common and remove dom dependence in common

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

Fixes #1990

src/common/Lifecycle.ts

@@ -3,7 +3,7 @@
  * @license MIT
  */
 
-import { IDisposable } from 'xterm';
+import { IDisposable } from './Types';
 
 /**
  * A base class that can be extended to provide convenience methods for managing the lifecycle of an