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>

test/benchmark/Event.benchmark.ts

@@ -4,7 +4,7 @@
  */
 
 import { perfContext, before, RuntimeCase } from 'xterm-benchmark';
-import { Emitter } from 'common/Event';
+import { Emitter } from 'common/primitives/Event';
 
 const ITERATIONS = 1_000_000;