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-webgl/src/renderLayer/BaseRenderLayer.ts

@@ -7,7 +7,7 @@ import { ReadonlyColorSet } from 'browser/Types';
 import { acquireTextureAtlas } from '../CharAtlasCache';
 import { IRenderDimensions } from 'browser/renderer/shared/Types';
 import { ICoreBrowserService, IThemeService } from 'browser/services/Services';
-import { Disposable, toDisposable } from 'common/Lifecycle';
+import { Disposable, toDisposable } from 'common/base/Lifecycle';
 import { CellData } from 'common/buffer/CellData';
 import { IOptionsService } from 'common/services/Services';
 import { Terminal } from '@xterm/xterm';