feat(plugin-selection): add auto-scroll on text selection

[wangtsiao]

Description

Adds auto-scroll functionality to text selection, enabling seamless multi-page selection when pointer approaches viewport edges.

Changes

• Auto-scroll triggers within 50-100px of viewport edges during text selection
• Uses requestAnimationFrame for smooth scrolling
• Automatically stops when selection ends or pointer moves away
• Supports all four directional scrolling (up, down, left, right)

Questions

Edge Threshold Configuration

I initially set all edge thresholds (top, bottom, left, right) to 50px.

However, when I was testing with the snippet example, I noticed that 50px wasn't quite enough for the top edge - the auto-scroll wouldn't kick in reliably when I moved my cursor near the top.

As a result, I increased only the top threshold to 100px while keeping the others at 50px:

private readonly AUTO_SCROLL_TOP_THRESHOLD = 100;
private readonly AUTO_SCROLL_BOTTOM_THRESHOLD = 50;
private readonly AUTO_SCROLL_LEFT_THRESHOLD = 50;
private readonly AUTO_SCROLL_RIGHT_THRESHOLD = 50;

This is my first PR here! I'm absolutely loving this project and it's been a pleasure working with the codebase. Please feel free to let me know if there's anything I should improve. Thanks so much for your time! 💙

[vercel]

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