GlyphsApp support

[rsheeter]

fontmake has a plugin @ https://github.com/schriftgestalt/FontMakeExportPlugin. Let's add fontc, for the time being marked as experimental.

fontMakeExport.glyphsFileFormat/Contents/Resources/plugin.py seems to just run the compiler on a temp file. We might eventually want to operate in memory but this seems like a very reasonable pattern to start with. fontc might actually be simpler than python as there isn't a venv to worry about.

[rsheeter]

I wonder if it makes sense to cdn compiler binaries such that the plugin can download them with a priori (baked in) knowledge of the expected size & checksum. That would require a bit of dilligence wrt releasing fontc but it might be worthwhile.

[rsheeter]

https://rust-lang.github.io/rustup/cross-compilation.html suggests we can compile e.g. a Mac binary on Linux which is very helpful. While GlyphsApp is Mac-only we should still publish at least Linux binaries to things like Google Sans CI can use them.

EDIT: rust-lang/rust#112501 seemingly contradicts ^. If we have to build on Mac that's not the end of the world