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/abstractScrollbar.ts

@@ -11,7 +11,7 @@ import { ScrollbarArrow, IScrollbarArrowOptions } from './scrollbarArrow';
 import { ScrollbarState } from './scrollbarState';
 import { ScrollbarVisibilityController } from './scrollbarVisibilityController';
 import { Widget } from './widget';
-import * as platform from 'common/Platform';
+import * as platform from 'common/base/Platform';
 import { INewScrollPosition, Scrollable, ScrollbarVisibility } from './scrollable';
 
 /**