add selectOnFocus prop to allow disabling selection on focus

[romansndlr]

Motivation

We want to allow a user to browse the list using their keyboard without "persisting" the selection.

This is consistent with many implementations out in the wild:

1. https://vercel.com/geist/multi-select
2. https://druids.datadoghq.com/components/facets/Facet

✅ 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. Render a GridList with selectionBehavior="replace" and selectionMode="multiple"
2. By default, focusing an item with keyboard should select it
3. Add selectOnFocus={false} to the GridList
4. Now focusing an item with keyboard should NOT automatically select it - selection should only occur on explicit press/click

<GridList
  selectionMode="multiple"
  selectionBehavior="replace"
  selectOnFocus={false}
>
  <GridListItem id="1">Item 1</GridListItem>
  <GridListItem id="2">Item 2</GridListItem>
  <GridListItem id="3">Item 3</GridListItem>
</GridList>

[romansndlr]

@snowystinger @LFDanLu

Would love your review on this one please 🙏🏻

[snowystinger]

Thanks for the PR, the team is currently on holiday. We'll review when we are back in the new year.
❤️

[snowystinger]

Would just using 'Option + Arrow Down/Up' to navigate via keyboard without selecting on focus work for you without needing these changes? Selection is pretty complex, so it'd be better if we didn't need to add another option.

[snowystinger]

should this be disregarded if selectionBehaviour is not replace?

[snowystinger]

how does one select multiple in replace? does shift+arrow down work?