fix: round to correct precision when step uses exponential notation

[lucasweng]

Closes #8274

✅ 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:

• Added tests for roundToStepPrecision.
• Existing tests should still pass.

🧢 Your Project:

[snowystinger]

maybe I'm over thinking this, but shouldn't this be

Suggested change

	expect(roundToStepPrecision(1.24, 0.1)).toBe(1.24);
	expect(roundToStepPrecision(1.24, 0.1)).toBe(1.2);

I realize it's the existing behaviour, I'll have to look back to remember why

[lucasweng]

Thanks for looking into this! I was wondering the same thing while looking at the original implementation 🤔

let precision = pointIndex >= 0 ? stepString.length - pointIndex : 0;

I also thought precision should maybe be stepString.length - pointIndex - 1, but the original behavior seemed correct, since all the existing test cases in NumberField.test and number.test passed. Also, this #8274 (comment) suggests that for an exponential notation like 1e-7, the precision should be 8. So, I implemented the existing behavior for exponential notation as well.