Add support for loading mediawiki.util from the top by default

• Introduces $wgPreloadJavaScriptMwUtil
• Instead of loading mediawiki.util as base module from the bottom, now loading it from queue position "top" if $wgPreloadJavaScriptMwUtil is true. And if false it'll remain in the bottom in practice as implied by other modules loading it as a dependency (i.e. mediawiki.page.ready depends on it)
• Fixes bug 33746