fix: first day of week bcp 47 parsing in middle of other extensions

[snowystinger]

Found in testing

Opted not to implement the entirety of https://unicode.org/reports/tr35/#unicode-locale-identifier
This particular change for the first day of the week fallback is only needed for FF and node right now.

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

unit test should cover it, but there is a story in RAC Calendar which sets a numbering system beyond the first day of the week via controls

🧢 Your Project:

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[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the documentation
• View the S2 docs

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the documentation
• View the S2 docs