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>

addons/addon-search/src/SearchEngine.test.ts

@@ -7,7 +7,7 @@ import { SearchEngine } from './SearchEngine';
 import { SearchLineCache } from './SearchLineCache';
 import { Terminal } from 'browser/public/Terminal';
 import type { ISearchOptions } from '@xterm/addon-search';
-import { DisposableStore } from 'common/Lifecycle';
+import { DisposableStore } from 'common/base/Lifecycle';
 
 function writeP(terminal: Terminal, data: string): Promise<void> {
   return new Promise(r => terminal.write(data, r));