Support for preconnect link relationship for Google Fonts

[ryanbalsdon]

Google Fonts asks you to include two preconnect links when using one of their fonts. The link tags when copied from the Google Fonts site look like this:

<link rel="preconnect" href="https://fonts.googleapis.com">
<link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
<link href="https://fonts.googleapis.com/css2?family=Roboto&display=swap" rel="stylesheet">

The rel attribute of links is parsed with a LinkType enum that is missing a few valid values. This LinkType enum is also used for a tags so a straight copy/paste into the enum might not be enough because not all types are supported in both tags!

[ryanbalsdon]

For a bit more context, here is the list of supported tokens for link relationships:
alternate, dns-prefetch, icon, manifest, modulepreload, next, pingback, preconnect, prefetch, preload, search, and stylesheet

And the same list of supported tokens for anchor relationships:
noreferrer, noopener, and opener

The Mozilla docs have a much larger list (some of these are not supported across both link and anchor elements though):
alternate, author, bookmark, canonical, dns, external, help, icon, license, manifest, me, modulepreload, next, nofollow, noopener, noreferrer, opener, pingback, preconnect, prefetch, preload, prerender, prev, search, stylesheet, tag

For context, here is the list currently supported (this does not distinguish between links and anchors):
alternate, author, bookmark, canonical, external, help, icon, license, manifest, modulepreload, next, nofollow, noopener, noreferrer, pingback, prefetch, preload, prev, search, shortlink, stylesheet, tag

Other than dns vs dns-prefetch, the Mozilla docs are a superset so probably are including historical or deprecated tokens? Looks like there are only about 4 currently missing in LinkType. I don't mind adding them in a small PR but would it be better to split these tokens into two separate lists (one for links and one for anchors)? Splitting into two lists might not be a backward-compatible change if anyone is currently using LinkType inside their anchor elements.