Add outerElementType/innerElementType props to Tree

[TrevorBurnham]

Summary

Lets consumers supply custom react-window outer/inner wrappers via props, without having to fork DefaultContainer. Also exports the default ListOuterElement, ListInnerElement, and DropContainer so a custom outer element can reuse the existing drop-target behavior.

When the new props aren't supplied, behavior is unchanged.

<Tree
  outerElementType={MyOuterWithStickyHeader}
  innerElementType={MyInner}
  ...
/>

This is the title feature cherry-picked out of #318. That PR also bundled a new padding model, a scrollTo rewrite, a useDndManager hook, and fork-publishing chores — each worth discussing separately, but none belong here. The author hasn't responded since 2025-10 and is shipping their own published fork, so I'm porting just the cleanly-scoped piece.

Closes #318 in spirit (credit to @jorgedanisc for the original).

Test plan

• yarn test passes in modules/react-arborist
• yarn build succeeds
• Spot-check that omitting both props still renders normally
• Spot-check a custom outerElementType that composes <DropContainer /> + {children} works

[Copilot]

Pull request overview

Adds opt-in customization points to Tree so consumers can supply custom react-window outer/inner wrapper components via props, while preserving current defaults when not provided. Also exposes the default list wrapper components publicly so custom wrappers can reuse existing behavior (e.g., drop-target overlay).

Changes:

• Extend TreeProps with outerElementType and innerElementType typed from react-window.
• Pass the new props through to FixedSizeList, defaulting to existing ListOuterElement/ListInnerElement.
• Export DropContainer, ListOuterElement, and ListInnerElement from the package public API.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
modules/react-arborist/src/types/tree-props.ts	Adds outerElementType/innerElementType to TreeProps for react-window wrapper customization.
modules/react-arborist/src/components/default-container.tsx	Wires the new props into FixedSizeList with nullish-coalescing fallbacks to current defaults.
modules/react-arborist/src/components/list-outer-element.tsx	Exports DropContainer so custom outer wrappers can reuse the existing overlay behavior.
modules/react-arborist/src/index.ts	Re-exports the default list wrapper components as part of the public API.

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