Meter: add support for informative variant

[reidbarber]

This adds the "informative" option to variant, and makes it the default. This is in line with Spectrum guidelines. Previously, variant was required and "informative" was not an option.

This is just a non-breaking Typescript change, since the correct styles were already there.

✅ 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:

Check storybook and docs.

🧢 Your Project:

RSP

[rspbot]

Build successful! 🎉

• View the storybook
• View the storybook-17
• View the storybook-16
• View the documentation

[rspbot]

Build successful! 🎉

• View the storybook
• View the storybook-17
• View the storybook-16
• View the documentation

[rspbot]

## API Changes

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@react-spectrum/meter

Meter

 SpectrumMeterProps {
-  variant: 'positive' | 'warning' | 'critical'
+  variant?: 'informative' | 'positive' | 'warning' | 'critical' = 'informative'
 }

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