Make css-loader use "preload" when available

[wincent]

In service of LPS-115876, the idea is to have the css-loader emit:

<link as="style" rel="preload" />

tags instead of:

<link rel="stylesheet" />

ones.

As noted here:

On top of that, preload does not block the window’s onload event, unless the resource is also requested by a resource that blocks that event.

In practice, we don't expect this to make page loads perceptibly faster, but it should at least avoid Lighthouse reporting onload events as delayed.

Related community Slack discussion: https://liferay-community.slack.com/archives/CB2QQ3M5F/p1598598237007500