Support building gasp table

[anthrotype]

Both Glyphs and DS+UFO sources allow to define the gasp table settings.

https://unifiedfontobject.org/versions/ufo3/fontinfo.plist/#opentype-gasp-table-fields

https://glyphsapp.com/learn/hinting-manual-truetype-hinting (search "gasp table" in the page)

This is an example of a .designspace project defining openTypeGaspRangeRecords in the fontinfo.plist:
https://github.com/wix-incubator/wixmadefor/

This one is a .glyphs project containing the equivalent gasp Table custom parameter:
https://github.com/justvanrossum/nabla

fontmake uses glyphsLib to convert the Glyphs custom parameter to the UFO property here:
https://github.com/googlefonts/glyphsLib/blob/main/Lib/glyphsLib/builder/custom_params.py#L527-L540

then ufo2ft reads the fontinfo's openTypeGaspRangeRecords to generate a gasp table here:
https://github.com/googlefonts/ufo2ft/blob/5fd168e65a0b0aa25298ad970546c3ab99d910c6/Lib/ufo2ft/instructionCompiler.py#L331-L342

It looks like write_fonts doesn't support writing gasp table yet, so that needs to be done first before fontc can even build one.

Right now, for font sources defining gasp settings ttx_diff.py reports:

COMPARISON
  Only fontmake produced 'gasp'