Make PREFETCH_DELAY customizable

[kinoute]

That would be great if we could change the value of PREFETCH_DELAY which is set by default to 100ms.

On some use cases, it is way too small. For instance a gallery with a lot of images with links on them, your cursor will obviously hover a lot of images while navigating and it will trigger a lot of prefetch for nothing.

[yujiteshima]

I've implemented this feature in PR #1483.

Changes:

• Added Turbo.config.drive.prefetchDelay configuration option (default: 100ms)
• The value is read dynamically via a getter, so changes take effect immediately

Usage:

// Increase delay for pages with many links (e.g., image galleries)
Turbo.config.drive.prefetchDelay = 300

This follows the existing pattern used by Turbo.config.drive.progressBarDelay.