feat: use requestIdleCallback for delayed script loading

[fkakatie]

Addresses discussion in #583 and relates to #376 and #408

This PR implements the requestIdleCallback approach proposed by @ramboz in discussion #583.

Changes

• Modified loadDelayed() to use requestIdleCallback when available
• Loads delayed scripts only when CPU has spare capacity or falls back to existing 3-second setTimeout for browsers without support (only ~82% coverage as of this comment)
• Prevents INP and TBT issues

Benefits

• Better performance: No artificial delays or user interaction requirements
• Predictable behavior: Loads for all visitors
• Progressive enhancement: Modern browsers get optimal loading, older browsers get current behavior
• Non-breaking: Consent handling remains separate, scripts in delayed.js unaffected

Test URLs:

• Before: https://main--aem-boilerplate--adobe.aem.live/
• After: https://idle-delayed-loading--aem-boilerplate--adobe.aem.live/

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Commits

• bc9d766 ✅ (latest)
• 8c923dd ✅

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