feat(Splitter): new component

[benjamincanac]

🔗 Linked issue

Part of #6662

Implements the widely requested Splitter component. Related closed requests: #6393, #6283, #6156, #1280, #305

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Adds a Splitter component for resizable panels, wrapping Reka UI's SplitterGroup, SplitterPanel and SplitterResizeHandle.

It follows the same items based API as Accordion and Stepper: each entry renders a panel and a draggable handle is inserted between them, with per panel content provided through dynamic slots. Panels support default, min and max sizes (percentage or pixel through unit), collapsible with collapsedSize, and the panel slot exposes collapsed, collapse, expand and resize for programmatic control. Orientation is set through orientation (mapped to Reka's direction), layout can be persisted with auto-save-id, and two dimensional layouts are achieved by nesting a Splitter inside a panel.

The PR includes the theme, a documentation page with examples, a playground page, and snapshot plus accessibility tests.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[pkg-pr-new]

npm i https://pkg.pr.new/@nuxt/ui@6670

commit: afdf75d