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/input/CompositionHelper.test.ts

@@ -6,7 +6,7 @@
 import { assert } from 'chai';
 import { CompositionHelper } from 'browser/input/CompositionHelper';
 import { MockRenderService } from 'browser/TestUtils.test';
-import { MockCoreService, MockBufferService, MockOptionsService } from 'common/TestUtils.test';
+import { MockCoreService, MockBufferService, MockOptionsService } from 'common/terminal/TestUtils.test';
 
 describe('CompositionHelper', () => {
   let compositionHelper: CompositionHelper;