Remove shared/ folder

We changed plans from moving web worker compatible code to shared/
to enforcing layers using core/, common/, ui/ and public/. This
change moves files back from the shared/ dir to clean up the
codebase.

Part of #1507

src/AccessibilityManager.ts

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