Skip JS preload hints when load-on-intent mode is active

[fballiano]

Summary

• The minification helper (sendEarlyHint) sends Link: rel=preload; as=script HTTP headers for every JS file during head rendering
• These preload headers instruct the browser to download all scripts immediately, completely bypassing the type="text/plain" load-on-intent deferral mechanism
• This fix skips sending preload hints for JS files when load-on-intent defer mode is active, preserving the intended behavior of deferring both download and execution until user interaction

Test plan

• Enable JS minification and load-on-intent defer mode
• Load a frontend page and check browser Network tab
• Verify JS files are NOT downloaded until user interaction (click, scroll, etc.)
• Verify CSS preload hints still work normally
• Verify JS preload hints still work when defer mode is disabled or set to defer-only

[fballiano]

Minor style nit: the two nested if statements could be a single condition with &&:

if ($as === 'script'
    && (int) Mage::getStoreConfig('dev/js/defer_mode') === Mage_Core_Model_Source_Js_Defer::MODE_LOAD_ON_INTENT
) {
    return;
}

PHP short-circuits &&, so getStoreConfig is still only called for script assets — same behavior, just cleaner.