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

package.json

@@ -74,6 +74,7 @@
     "lint:fix": "turbo run lint:fix",
     "build:wasm": "cd packages/rust/engine && wasm-pack build --dev --out-dir pkg",
     "build:wasm:release": "cd packages/rust/engine && wasm-pack build --release --out-dir pkg",
+    "build:rapier2d": "node scripts/build-rapier2d.mjs",
     "copy-modules": "node scripts/copy-engine-modules.mjs"
   },
   "author": "yhh",