feat(Slider): Add slider thumb positioning prop

[jpenilla]

Looking for feedback before I add docs - not sure if this property should be moved to the thumbs themselves somehow

Motivation:

With an equal padding around the slider, the containment makes alignment look 'off'.

[github-actions]

built with Refined Cloudflare Pages Action

⚡ Cloudflare Pages Deployment

Name	Status	Preview	Last Commit
bits-ui	✅ Ready (View Log)	Visit Preview	b2e7ef0

[huntabyte]

I like this idea!

[changeset-bot]

⚠️ No Changeset found

Latest commit: b2e7ef0

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR