Use font-sans as the default font, not the system font

[adamwathan]

We get so many questions about this on Discord ("I changed my font but it's not showing on my site") that I think it makes sense to change this so the behavior is more intuitive.

Could probably call this a breaking change, but it would literally only affect people who had changed font-sans to a different font but wanted the system UI font to be the default font for their site (only using the font-sans class in select places to purposely deviate from the system font occasionally).

I think I feel comfortable betting $500 that that is literally nobody. I think in hindsight the two most intuitive behaviors for this are:

1. Do what we're doing in this PR.
2. Do literally nothing, so the user just sees sans-serif by default, so when they look at dev tools at least it's obvious that it's not that their change to font-sans isn't taking effect, since sans-serif doesn't match the value they were trying to change either.