fix: allow underscores and mixed-case PaymentSheet font names on android

[iM-GeeKy]

Summary

• Relaxed Android Payment Sheet font-name validation so res/font filenames can include underscores and mixed-case letters (for example Inter_Bold, Nunito_400Regular, other google fonts), instead of only lowercase alphanumeric characters.
• Updated the thrown error and TypeScript docs to match the new allowed characters

Motivation

• Closes Font names with underscores incorrectly throw an error on Android in PaymentSheet #1636
• Closes Allow underscores in font names on Android for PaymentSheet appearance #1637
• Closes Android custom fonts with expo #1758

Testing

• I tested this manually
• I added automated tests

Documentation

Select one:

• I have added relevant documentation for my changes.
• This PR does not result in any developer-facing changes.

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[iM-GeeKy]

@porter-stripe Would you or another team member mind taking a look at this? It would be much appreciated. I tagged you as you were one of the ones to last update the repository.