Font Library: add a way to create system fonts

[creativecoder]

What problem does this address?

Sometimes users want to define a system font, that uses a font stack like

font-family: Constantia, "Lucida Bright", Lucidabright, "Lucida Serif", Lucida, Georgia, serif;

There's currently no way to do this from the font management UI.

What is your proposed solution?

In addition to uploading font files, take input for creating a system font stack that defines a theme.json fontFamilies entry without fontFace.

There could be a font collection of recommended web safe system font stacks.

[matiasbenedetto]

This issue closely related to this one: #54186