Fix: Transpile private class fields for ES2019 runtime compatibility

[Copilot]

TypeScript private class fields (#privateProperty) weren't being transpiled, causing runtime errors in Lynx's ES2019-based JavaScript engine.

Changes

• SWC Configuration: Added target: 'es2019' to global SWC config to transpile private fields to WeakMap-based implementation
• Test Coverage: Added test verifying private fields transpile correctly (no # syntax in output, WeakMap pattern present)
• Demo: Updated example app to demonstrate private property usage

Technical Details

Without an explicit target, SWC defaults to modern ES output, leaving private field syntax unchanged:

// Input
class Demo {
  #private: string;
  getPrivate() { return this.#private; }
}

// Before: Output for Lynx runtime (unsupported)
class Demo {
  #private;  // Runtime error: unsupported syntax
}

// After: Output for Lynx runtime (ES2019 compatible)
var _private = new WeakMap();
class Demo {
  constructor() { _private.set(this, void 0); }
  getPrivate() { return _private.get(this); }
}

The main thread layer already set this target explicitly, but the background (Lynx runtime) layer inherited global config without a target.

Original prompt

在 @lynx-family/lynx-stack/files/examples/react/src/App.tsx 创建一个带有私有属性（#privateProperty）的demo类并测试使用，然后运行时引擎会报错并不支持，因此希望你看看为啥。

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