fix: exclude fonts.css from the base stylesheet

[huyenltnguyen]

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fonts.css references the font files in /assets. This works with the component library itself, but is broken if used in a consumer project as the fonts are not included in the library bundle.

ui/src/fonts.css

Line 5 in c754cee

src: url("./assets/fonts/lato/Lato-Light.woff") format("woff");

I'm keeping the fonts.css out of base.css for now, and not export the fonts.css out for consumption. I'll add it back into base.css via #123, to export the stylesheet together with the font files.