Move common primitives to src/common/primitives/

Phase 1 of #5963: reorganize building blocks (buffer, parser, input,
data, and shared utilities) under common/primitives/. Runtime code
(services, public API, CoreTerminal, InputHandler) stays in place.

Update import paths across browser, headless, addons, and tests.
Add TypeScript path mappings so common/* aliases resolve primitives
first, then the common root.

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

demo/client/components/window/cellInspectorWindow.ts

@@ -7,7 +7,7 @@ import { BaseWindow } from './baseWindow';
 import type { IControlWindow } from '../controlBar';
 import type { IBufferCell } from '@xterm/xterm';
 
-// Underline style values from common/buffer/Constants.ts
+// Underline style values from common/primitives/buffer/Constants.ts
 const enum UnderlineStyle {
   NONE = 0,
   SINGLE = 1,