In default static fonts, remove alts before or after build, to avoid needless filesize bloat

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Currently, two alts are added for most glyphs, to allow bouncy & irregular axes.

However, for the static, normal fonts, these alts will not serve a purpose.

Include a build step that will cut these out, to make these files smaller.

• add subset calt step
• check that fonts work as expected (esp. kerning and other features like fractions)

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A comparison of the file sizes:

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Looks good with 7a9c6fc! The subset fonts are smaller files than before, but keep all the previous functionality.