Restructure common into layered folders (issue #5963)

Reorganize src/common into platform (base, encoding, data, buffer,
parser, input), integration (services, terminal), and public API layers.

- Move core utilities to common/base/
- Move TextDecoder to common/encoding/
- Move Types, CircularList, Color, TaskQueue, SortedList to common/services/
- Move CoreTerminal, InputHandler, WindowsMode to common/terminal/
- Define ICharset in common/data/Charsets.ts (re-exported from Types)
- Update imports across src/, addons/, and test/
- Add per-layer tsconfig.json files and REARCHITECTURE.md
- Root common/tsconfig.json remains the composite build entry

Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>

src/browser/scrollable/scrollbarArrow.ts

@@ -5,7 +5,7 @@
 
 import { GlobalPointerMoveMonitor } from './globalPointerMoveMonitor';
 import { Widget } from './widget';
-import { TimeoutTimer } from 'common/Async';
+import { TimeoutTimer } from 'common/base/Async';
 import * as dom from '../Dom';
 
 export interface IScrollbarArrowOptions {