SVG font support

[nulano]

FreeType 2.12 (#6169) adds support for color fonts in SVG format.

From freetype/freetype@21d0fa3:

FreeType now handles OT-SVG fonts, to be controlled with
FT_CONFIG_OPTION_SVG configuration macro. By default, it can
only load the 'SVG ' table of an OpenType font. However, by using
the svg-hooks property of the new 'ot-svg' module it is possible
to register an external SVG rendering engine. The FreeType demo
programs have been set up to use 'librsvg' as the rendering
library.

It would be nice to add support for this to Pillow.
librsvg appears to be LGPL-2.1 licensed, so a wrapper similar to that used for FriBiDi/Raqm may be needed.
This is closely related to loading SVG images in Pillow, it may be possible to add support for both at once: #3509

For reference, the freetype demo callbacks are here: https://gitlab.freedesktop.org/freetype/freetype-demos/-/blob/master/src/rsvg-port.c