fix(web-components): compact empty vertical dividers

[Cody Sorgenfrey (codysorgenfrey)]

Summary

• Track meaningful default-slot content rather than relying on :empty, which does not match whitespace-only light DOM.
• Apply the 20px compact minimum to empty vertical dividers while retaining the 84px contentful minimum and inset/alignment styling.
• Add Playwright regression coverage for empty, whitespace/comment-only, contentful, and dynamically changing slots; update CSR/SSR styles, templates, generated API documentation, and the patch change file.

Fixes #31623

Validation

• Immutable Yarn install succeeded through the Microsoft npm proxy (YARN_NPM_REGISTRY_SERVER=https://packagefeedproxy.microsoft.io/npm/).
• nx run web-components:type-check and nx run web-components:lint passed (lint has 97 existing warnings).
• Focused nx run web-components:e2e -- --grep Divider passed: Chromium CSR/SSR 22/22 and WebKit CSR/SSR 22/22.
• nx run web-components:check:ssr verified the divider outputs but reports three pre-existing stale templates elsewhere: textarea, text-input, and field.
• Firefox browser runs stalled at startup on this host and could not be completed.