Create a definition for getting fonts from the system

[jakearchibald]

Right now the spec says "For each local font font on the system".

I think it'd be better to create definitions for this. Eg:

Font metadata has:

• A postscript name, a DOMString.
• A full name, a DOMString.
• A family, a DOMString.

A system font is a font that is available system-wide by the operating system.

To read a system font as a font representation… (this lets you detail the kinds of filtering a browser may perform here)

To get all system font metadata, perform the following steps:

1. Let |fonts| be a subset of [=system fonts=] [=read as font representations=]. Subsetting may be defined by the user agent, and can include safelists, or a selection UI shown to the user.
2. Let |fontMetadatas| be a new list.
3. [=For each=] |font| of |fonts|:
   1. Let |metadata| be a new [=font metadata=].
   2. Set |metadata|'s [=postscript name=] to…
   3. Etc etc
4. Sort |fontMetadatas| by…
5. Return |fontMetadatas|.