Prepend the given element before the first existing element of same type.
This is generally used for loading Docma assets first (such as docma-web.js
and docma.css). Update: this is NOT used for scripts anymore, bec. jQuery
2.x duplicates the scripts. For scripts, we use our
`HtmlParser.DOM.insertBeforefirst()` method instead.
@private

@param {jQuery} $container - Container jQuery element.
@param {String} selector - Target element type selector. e.g. `"script"`.
@param {jQuery} elem - Element to be prepended.
function _insertBeforeFirst($container, selector, elem) {
    const foundElems = $container.find(selector);
    if (foundElems.length > 0) {
        foundElems.eq(0).before(elem);
    } else {
        $container.append(elem);
    }
}