[feat]: Create `<pf-v6-tree-view>` element

[bennypowers]

Summary

Create a <pf-v6-tree-view> web component implementing the PatternFly v6 Tree view component.

Hierarchical tree display for nested data, with expandable nodes, checkboxes, and action menus.

Prior Art

Source	Element	Status
PFE v4	--	--
cem	<pf-v6-tree-view>	Exists
cockpit	<pf-v6-tree-view>	Exists
chickadee	--	--

Sub-components

• <pf-v6-tree-item> - Individual tree node

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with @patternfly/react-core TreeView
• MAY adjust element API to leverage web platform strengths
• SHOULD maintain CSS custom property theming compatibility
• SHOULD expose useful CSS shadow parts