WIP: Add `@font-face`

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This code is currently in-progress and should not be merged

This PR will add @font-face support for the most commonly used attributes:

• font-family
• src
• font-variant
• font-weight
• font-style

It does not implement unicode-range, font-feature-settings, or font-stretch. Other authors are encouraged to add these in later.

Questions for elm-css core team

There already exist functions called fontFamily and src in elm-css. They seem to be incompatible with the semantics of @font-face. To wit:

fontFamily takes only FontFamily a, which only allows for serif, sansSerif, monospace, cursive and fantasy. In this case, we're defining a brand new font with a new name. The docs suggest using fontFamilies if you want custom-named fonts, but that doesn't make sense here because we're defining one font name.

src is of type ImportType compatible -> String, and I just really don't know what to do about that.

To see what an ideal implementation would look like, see the proposed fixture.