new_audit: add preload-fonts audit

[lemcardenas]

Summary

This PR addresses #11227 and adds a new audit that validates if all fonts that use font-display: optional were preloaded. Also adds a smoke test for this audit in the perf smoke test suite.

Related Issues/PRs

#11227

[paulirish]

as mentioned. i went deep on this over here: #11227 (comment)

i think the audit as currently configured doesn't as much impact we wanted. Because 1) few people are using optional 2) preload is valuable to all webfonts 3) the documented optimizations don't have much to do with preload. :/

So we'll have to get consensus on the change there first. Luckily, I think we'll retain most of this PR anyhow.

[paulirish]

So we'll have to get consensus on the change there first. Luckily, I think we'll retain most of this PR anyhow.

Okay! we can move forward again. We can land this PR with the given scope. It's currently quite narrow, but in order to broaden it, we need a few questions answered first. After that, we may tweak this audit.

All this investigation and discussion also means we'll probably tweak the naming/framing of the audit. I've left those suggestions inline below.

[paulirish]

i had a concern that findFontDisplayDeclarations is rather expensive. and was thinking we could skip it if we saw no font network requests.

but.. i looked at some timings on LR for lh:audit:font-display (which also does that work) and it seems plenty quick

90pctl is 12ms! fast.

so yeah ... i'm cool with this cost.

[lemcardenas]

Hooray for preventative perf checks 💯