metrics, unpack: Calculate and expose the xWidthAvg metric

[michaeltaranto]

Calculate the average width of lowercase characters and expose as xWidthAvg. This value facilitates calculating better font fall backs, as it enables calculating the size-adjust CSS property (see the createFontStack PR).

Why a custom metric?

There is a built-in value called xAvgCharWidth in the OS/2 table, however the definition of its value has changed over time and is currently the average of all glyphs from a font file. Instead, we calculate an average width of the glyphs based on their frequency of occurrence. From testing this has proved to be quite effective.

The category field

The category field exists on every Google Font and describes the font style, e.g. “serif”, ”sans-serif”, etc. We are exposing this (as well as populating it manually for system fonts), as frameworks like NextJS are using it to select which system font to fallback to.

Notes

• Align the FontMetrics type from core with shape of metrics from metrics package.
• Updates to the latest Google Fonts
• Add sections with links to metrics and unpack from top level readme

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🦋 Changeset detected

Latest commit: f74ddec

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@capsizecss/metrics	Minor
@capsizecss/unpack	Minor
@capsizecss/core	Patch

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