Add alternate VF instances for different design space slices

Add 6 new variable font instances to MonaSans.glyphspackage:
- MonaSansVF[wdth,opsz,wght] — upright only, all axes except ital
- MonaSansVF[wdth,wght] — upright text only (no opsz, no ital)
- MonaSansVF-Italic[wdth,opsz,wght] — italic only, all axes except ital
- MonaSansVF-Italic[wdth,wght] — italic text only (no opsz, no ital)
- MonaSansVF[opsz,wght] — normal width upright only
- MonaSansVF-Italic[opsz,wght] — normal width italic only

Each instance disables the appropriate masters and has exports=0
(for Glyphs.app export only, not gftools builder).

For the two instances without opsz ([wdth,wght] variants), added
Replace Feature for rlig (without opsz condition) and Remove Features
for kern (which uses opsz variation values) to avoid axis errors.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: kylewaynebenson