Add support for `loadFont`

Sorry if I'm missing something, but it's my understanding that `loadFont` is not part of the `jimp-native` library. I've noticed that bitmap font loading is *very slow* in `jimp`, and it looks like even if you try to run a bunch of font loading concurrently, they all get stuck in a queue anyway and the situation doesn't improve.

I think we'd see huge gains by implementing the font loading natively (and in parallel!) and this would eliminate a huge bottleneck in terms of speed.

No files, tests, or entry points are named. Start by locating the native font-loading boundary and Jimp's bitmap-font loading path, then determine how concurrent loads are currently queued. Done means `loadFont` is supported natively and the reported font-loading bottleneck is addressed.

Written by the indexing model from the issue text.