Do not build list of available slots when no error

[jorrit]

An error was logged on my website because Intl.ListFormat was not available:

TypeError: undefined is not a constructor (evaluating 'new Intl.ListFormat()')
  at c(./node_modules/react-aria-components/dist/utils.mjs:76:1)
  at m(./node_modules/react-aria-components/dist/utils.mjs:87:1)

It seemed the site was accessed using Playwright, an automated browser. (https://github.com/microsoft/playwright, microsoft/playwright#23978).

Out of curiousity, I checked which react-aria code was using Intl.ListFormat and it seemed that the code in utils that caused this should not have run, as the list is only needed in case of errors.

I refactored the code so the list of available slots is only compiled in case the requested slot is not available. This is a small speed optimization, as there is no longer a new instance of Intl.ListFormat on every render.

✅ 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:

1. Find a browser with no support for Intl.ListFormat.
2. Create a component with subcomponents that use slots.
3. Use an valid slot name.
4. Notice that there is no error because missing Intl.ListFormat.

[snowystinger]

thanks for the optimization!