Use `fallback` property in development mode

[selipso]

I'm not sure if this happens in production builds or not, but I noticed that when developing offline using a Google Fonts url as a font, the astro compiler throws an error instead of using the fallback default sans-serif font.

Steps to reproduce:

• turn off wi-fi / networking on your computer
• astro dev a project that uses a google font URL in the AstroFont component
• Compiler throws error

Expected behavior:

• Project should compile using the default sans-serif font if network is unreachable