Refactor for simplicity

Use a unified `client` for both calling methods in main and listening to events in renderer:

```ts
import { createClient } from '@egoist/tipc/client'

// MainHandlers for calling methods defined in main process
// RendererHandlers for listening to events and invoke the handler in renderer process
export const client = createClient<{ MainHandlers, RendererHandlers }>({
on: window.ipcRenderer.on
})
```

```ts
client.main.someHandlerInMain(arg1, arg2)

client.renderer.someHandlerInRenderer.on(() => {
// ...
})
```