refactor: Handle system and google font inputs uniformly

[michaeltaranto]

In preparation for supporting metrics for font variants (eg. weight and style), this refactors how metrics are extracted and built internally.

Key goals were:

• Normalise system fonts and google fonts data structures
• Pre-extract metrics from google fonts
• Generate script can just read(not extract) metrics for all font sources

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Latest commit: 04db46e

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[askoufis]

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