Build and package WOFF2 webfonts

[alerque]

Here are webfont builds! @lutetiensis, @CoreDump2020, and of course anybody else interested in using Libertinus on the web — I would love to hear feedback if you can actually give the woff2 files a shakedown in your actual use cases.

You can find the files under the 'Checks' tab of this PR. The Fontship check should have artifacts attached: Libertinus-7.000-r14-gbc8b13.zip

Closes #363
See also #362

I haven't tested these yet myself beyond making sure they seem to be font files with shapey things in them. Note the overall size savings over OTF is 20-50%, plus the way the file is arranged internally helps browsers make use of some of the data early before it even has the whole file.

[lutetiensis]

I dirtily pushed everything on my fork (here). You can directly access the files using jsDelivr, simply change the name of the woff2 file you want to use.:

https://cdn.jsdelivr.net/gh/lutetiensis/libertinus@master/releases/Libertinus-7.000-r14-gbc8b13/static/WOFF2/LibertinusSerif-Regular.woff2

@alerque I haven't experienced any issue so far on Regular, Italic, Bold & BoldItalic. Good job!

[alerque]

See this comment on the CDN issue, but I think in addition to adding WOFF files to the release package here we should publish an official package to NPM with just the webfonts and supporting files. See also theleagueof/fontship#88

[alerque]

I am waiting to close this until the OTF→WOFF2 target is part of a tagged Fontship release. The current stable tag only works for TTF→WOFF2 and we're not currently generating TTFs (see #326). This PR currently builds using the master branch, but I'd like to see it build using the latest tagged release before I merge this.

In the mean time if anyone else checks the WOFF2 files in real usage it would be great to hear confirm/deny comments about how they work.