docs(editor): improve editor-app build documentation and add build:rapier2d script

- Add `pnpm build:rapier2d` command to automate Rapier2D WASM build process
- Fix gen-src.mjs path to correctly locate thirdparty/rapier.js
- Update init.ts to work with new wasm-pack web target (auto-initialization)
- Fix behavior-tree-editor build config for asset-system dependency
- Update README_CN.md and README.md with simplified build instructions

packages/physics/rapier2d/scripts/gen-src.mjs

@@ -14,7 +14,7 @@ import { fileURLToPath } from 'url';
 
 const __dirname = dirname(fileURLToPath(import.meta.url));
 const packageRoot = join(__dirname, '..');
-const rapierRoot = join(packageRoot, '..', '..', 'thirdparty', 'rapier.js');
+const rapierRoot = join(packageRoot, '..', '..', '..', 'thirdparty', 'rapier.js');
 const srcTsDir = join(rapierRoot, 'src.ts');
 const src2dDir = join(rapierRoot, 'rapier-compat', 'src2d');
 const outputDir = join(packageRoot, 'src');