fix(mono): maintain fixed width for fractions and code ligatures

[wayne-shih]

Fixes #33

Rationale and fixes implemented

Few things to note here, this PR tweaks all the mono glyphs to the same metric width (600) -- thus, system and code editors will recognise Geist Mono as a monospace font.

In doing so, a few things needed to be changed :

• Fractions
  • Previously 1200 wide, this is changed to 600 wide as each are individual unicode glyph rather than a custom ligature
  • NOTE: It has been redrawn to fit 600 wide ; please review from a stylistic point of view, I'm happy to remove this if you'd like to redraw it differently yourself
  • Removed frac open type feature. It didn't make sense to include it as the input was 3 characters (ie. 1 / 2) to result in 1 character (½)
• Code ligatures
  • Previously these were various widths (multiple of 600; ie. 1200, 1800, 2400, 3000), these have all been changed to fixed 600.
  • To make the code ligature still work, I introduced a SPC glyph -- empty character -- which is used as a filler character to construct the ligature
  • Disabled liga (standard ligature) feature -- previously auto generated
  • Added a new calt (contextual alternative) feature -- this replaces the liga and contains all previous instructions but utilising the SPC character. This is the same/similar approach as how JetBrains Mono, Fire Code, et al... uses.

    lookup asciitilde_asciitilde_greater.liga {
        sub SPC SPC greater' by asciitilde_asciitilde_greater.liga;
        sub SPC asciitilde' greater by SPC;
        sub asciitilde' asciitilde greater by SPC;
    } asciitilde_asciitilde_greater.liga ;
    

Test export files -- ttf only

8f30371GeistMono.zip

Note
This is a direct export from Glyphs -- not using fontmake or any other undocumented flows you might have in place already

Supporting screenshots

Redrawn fraction glyphs

Now recognised as a monospace by WebStorm
(I didn't test other code editors or systems, would be super surprised if others don't recognise it now as a monospace)

Ligature still works

Actions and next steps

Keen to get some peer (/stress) testing in to see if I'm missed any code ligature pairings and if any ignore sub needed to be added.

Fractions should be viewed from a design perspective.

If all happy, would need to cut a new release -- I didn't do this bit as it's not documented how y'all are exporting the binary font files (either directly from Glyphs or using fontmake or other scripts)