enhancement: adding firstIndex and lastIndex to virtual

[davedbase]

• packages/virtual/src/index.tsx: added firstIndex: numberandlastIndex: number | undefinedtoVirtualListReturn's type and the returned object. lastIndexislastIdx - 1(since slicing is exclusive) andundefined` for an empty list.
• packages/virtual/test/index.test.tsx: added the equivalent test cases from the PR (basic values, scroll updates, overscan, bottom-of-list, empty list, singleton list), adapted to this repo's createRoot/flush test pattern for Solid 2.0.
• Added a changeset (patch bump for @solid-primitives/virtual).

[changeset-bot]

🦋 Changeset detected

Latest commit: 66bd829

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@solid-primitives/virtual	Patch

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