feat: appkit exposed apis

[MarioCadenas]

Summary: Plugin SDK Pattern for Exposed APIs

Overview

This PR introduces an explicit SDK pattern for exposing plugin APIs, replacing direct plugin instance access with a controlled public interface.

Key Changes

1. SDK Pattern for Plugin APIs

• Plugins now define their public API via an optional exports() method
• AppKit wraps each plugin's exports with asUser() automatically
• Methods are bound to the plugin instance, ensuring correct this context when destructured

2. Architecture Improvements

• Removed static singleton (_instance) - each createApp() returns a fresh instance
• Changed to ES private fields (#pluginInstances, #setupPromises) for true runtime privacy
• Centralized asUser handling in AppKit instead of duplicating in each plugin

3. Type System Updates

• Added PluginSDK - extracts SDK type from plugin's exports() return type
• Added WithAsUser - wraps SDK with asUser method signature
• Updated PluginMap to return WithAsUser<PluginSDK<...>> instead of raw plugin instances

4. TypeScript Configuration

• Updated target and lib to ES2022 (enables Object.hasOwn())

5. Changes to SDK usage of AppKit

• appkit.pluginName now returns the SDK object, not the plugin instance
• Plugins that want to expose methods must implement sdk() (optional, defaults to {})

Example

class MyPlugin extends Plugin {
  myMethod() { /* ... */ }
  
  exports() {
    return {
      myMethod: this.myMethod,
    };
  }
}
-----

const appKit = await createApp({
  plugins: [myPlugin()]
})

appKit.myPlugin.myMethod()
appKit.myPlugin.asUser(req).myMehtod()

[pkosiec]

It looks awesome now👌 great job! just a small comment

(BTW I don't have any better name than sdk but it sounds a bit broad, maybe you'll find some naming alternatives?)