Allow preloading of Sass elements

[John-Toohey]

Trunk can build scss and sass files with <link rel="scss"... however it cannot preload them.

When I wrote:

<link rel = "preload" as = "scss" href = "styles/noncritical.scss" data-trunk/>

Trunk errored out:

ERROR ❌ error
error from HTML pipeline

Caused by:
    unknown <link data-trunk .../> attr value `rel="preload"`; please ensure the value is lowercase and is a supported asset type

I really like trunk and I think this feature would make it a lot nice for larger projects which have a non critical sass aswell as inlined critical sass!