Add Postscript blue zones for letters and numbers

[madig]

Summary of the Pull Request

Add Postscript blue zones that enclose letters and common numbers and define vertical and horizontal stem widths. This helps with autohinting OTFs.

PR Checklist

• Closes #xxx
• CLA signed. If not, go over here and sign the CLA
• Requires FONTLOG.txt to be updated
• Requires /images/cascadia-code.png and/or /images/cascadia-code-characters.png to be updated
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

Detailed Description of the Pull Request / Additional comments

When not producing TTFs but OTFs, hinting is a matter of running https://github.com/adobe-type-tools/psautohint/ on the resulting binary. For the autohinter to do its job, it needs to know the standard widths of a font plus so-called blue zones, which tell the tool which heights points of a glyph should be snapped to. They also help with consistency in the design phase, because e.g. Glyphs will indicate when a point is within or outside a blue zone.

TTFs are unaffected by this change since the format does not use them.

Before (if applicable) and After Images of the Character(s)

Validation Steps Performed

Manually looking at the resulting font in a terminal and in VSCode.

[mdtauk]

PS Hinting for OTF is separate to TTF hinting.

[madig]

Yes. This is for my private OTFing/in anticipation for making official OTFs one day. I love the way FreeType renders them.

[DHowett-MSFT]

Hey @aaronbell -- since we're not producing our own OTFs, should we merge this to make it so that other people can? I feel bad having let this languish for so long.

[aaronbell]

@DHowett-MSFT Sure. I haven't had a chance to review the additions, but I figure @madig got it right :).

It might be worth considering building our own OTF files, though. ufo2ft also has a makeOTF function, so it shouldn't be too hard to add it into the build script.

[madig]

I've been using these in my home-made OTF and everything looks great. So, unless someone changed fundamental design heights, they should be good.

[DHowett-MSFT]

@madig thank you; I'm sorry it took so long to merge this. 😄

[madig]

No worries, this PR paid dividends in the meantime.