Fixes to <VariantSelector> API and add useVariantUrl hook

[blittle]

Fixes to <VariantSelector> API and add useVariantUrl hook.

1. <VariantSelector> now takes handle as a property. This makes the component useable on pages other than the default PDP route.
2. <VariantSelector> also gives you the product options search param, so if you have a custom PDP route, you can still link to it.
3. The useVariantUrl is useful for situations where you want to get a link to a product variant.

WHY are these changes introduced?

Fixes #0000

WHAT is this pull request doing?

HOW to test your changes?

Post-merge steps

Checklist

• I've read the Contributing Guidelines
• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've added a changeset if this PR contains user-facing or noteworthy changes
• I've added tests to cover my changes
• I've added or updated the documentation

[github-actions]

We detected some changes in packages/*/package.json or packages/*/src, and there are no updates in the .changeset.
If the changes are user-facing and should cause a version bump, run npm run changeset add to track your changes and include them in the next release CHANGELOG.
If you are making simple updates to examples or documentation, you do not need to add a changeset.

[blittle]

I run all of these before commit. So figured I'd add a simple check command.

[blittle]

We can't return a real URL object, because we don't easily have the origin at the moment, because useLocation() doesn't return it. I felt like path or pathname implies it doesn't include the search (because the native URL.prototype.pathname doesn't).