[feat] Add tauri://pageLoad event listener support in frontend

[KazeLiu]

Describe the problem

When creating a new window using WebviewWindow, I can only use tauri://created to listen for window creation. However, this event triggers much earlier than the actual page load completion.

Currently, if I need to inject JavaScript after the page is fully loaded, I have to:

1. Use WebviewWindowBuilder::new(&app_handle, &label, url_parse).on_page_load in the backend
2. Emit an event to notify the frontend

The challenge is that I'm not familiar with backend development in Rust, and I have to rely on my friend to help implement these backend methods.

Technical Context:

• Current approach requires backend implementation
• Frontend developers need a more accessible way to handle post-load operations
• Similar to how browsers provide window.onload event

thanks

Describe the solution you'd like

1. Add a frontend event listener for page load completion (similar to tauri://pageLoad)
2. Add frontend API for JavaScript injection

This would greatly improve the developer experience for frontend developers who are not yet proficient in Rust but need these functionalities for their projects.

Alternatives considered

No response

Additional context

No response