Update to modern standard for scrollbar styles

[endigo9740]

Linked Issue

Closes #2538

Description

Migrates the Skeleton plugin base styles from the -webkit- vendor prefix to the new modern standards described here:

• https://developer.mozilla.org/en-US/docs/Web/CSS/scrollbar-color
• https://developer.chrome.com/docs/css-ui/scrollbar-styling

This matches the exact implementation in Skeleton v3, which better prepares for the transition going forward.

Changsets

Instructions: Changesets automate our changelog. If you modify files in /packages, run pnpm changeset in the root of the monorepo, follow the prompts, then commit the markdown file. Changes that add features should be minor while chores and bugfixes should be patch. Please prefix the changeset message with feat:, bugfix: or chore:.

Checklist

Please read and apply all contribution requirements.

• This PR targets the dev branch (NEVER master)
• Documentation reflects all relevant changes
• Branch is prefixed with: docs/, feat/, chore/, bugfix/
• Ensure Svelte and Typescript linting is current - run pnpm ci:check
• Ensure Prettier linting is current - run pnpm format
• All test cases are passing - run pnpm test
• Includes a changeset (if relevant; see above)

[changeset-bot]

🦋 Changeset detected

Latest commit: 9304823

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

This PR includes changesets to release 1 package

Name	Type
@skeletonlabs/tw-plugin	Minor

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