feat: add React Native runtime for running CTS tests

[wcandillon]

Add a React Native compatible runtime that works with bundlers that don't
support dynamic imports (like Metro).

The runtime uses pre-generated static imports instead of dynamic import().
Projects need to generate an all_specs.ts file at build time using the
provided gen_rn_specs.ts tool or equivalent.

Files added:

• src/common/runtime/rn/loader.ts - ReactNativeTestFileLoader class
• src/common/runtime/rn/runtime.ts - CTSRunner class and utility functions
• src/common/runtime/rn/index.ts - Public exports
• src/common/runtime/rn/README.md - Documentation and usage examples
• src/common/tools/gen_rn_specs.ts - Tool to generate static imports

Usage:

import { CTSRunner } from './common/runtime/rn';
import { allSpecs } from './common/runtime/rn/generated/all_specs';

const runner = new CTSRunner(allSpecs, { debug: false });
const { summary } = await runner.runTests('webgpu:api,operation,adapter,*');

Note: React Native requires polyfills for Event, EventTarget, and MessageEvent.
See README.md for details