Move platform to common and remove dom dependence in common

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

Fixes #1990

src/renderer/atlas/CharAtlasGenerator.ts

@@ -4,7 +4,7 @@
  */
 
 import { FontWeight } from 'xterm';
-import { isFirefox, isSafari } from '../../core/Platform';
+import { isFirefox, isSafari } from '../../common/Platform';
 import { IColor } from '../Types';
 import { ICharAtlasConfig, CHAR_ATLAS_CELL_SPACING } from './Types';