List available fonts?

## Issue or Feature

Is it currently possible to get a list of the available fonts? - i can't see an applicable api...
It seems this should be possible in pango using pango_font_map_list_families (); I understand it's not a feature of HTML5 canvas, but we already have some extended features, including RegisterFont in node-canvas.

If it's not currently possible, is it something which others are interested in? Certainly for me, as both an aid to giving a user an informed choice, and as a diagnostic aid when display is not as expected, this would be a really useful feature; and on the face of it does not seem to be terribly difficult to implement. If you've looked at it before and rejected it as difficult, then please don't let me plough on and try without discussion :).

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## Steps to Reproduce

## Your Environment
latest version of canvas, Windows, MAC, Linux

Start with the existing RegisterFont API and the node-canvas installation context, then investigate the Pango pango_font_map_list_families() entry point mentioned in the issue. Done means providing a way to retrieve available font families on Windows, macOS, and Linux, with the API shape and behavior agreed on first.

Written by the indexing model from the issue text.