[BIX-8750]: Accept nullable block-skeleton scroll refs

[srubin]

Summary

React 19 no longer includes null implicitly in RefObject<T>. Draft.js's block-skeleton options therefore reject the nullable scroll refs that Descript passes, although the hook already handles null by falling back to the editor contents element.

Accept RefObject<HTMLElement | null> in both options. This works with React 18 and React 19 without a custom compatibility type. Extend the existing skeleton-promotion test to exercise both mounted and null scroll refs. Production JavaScript is unchanged.

Related to BIX-8750 and descriptinc/descript#45317. No package release or Descript dependency bump is included.

Test Plan

• Changed-file ESLint.
• Editor suite: 14 tests passed.
• Related suites: 28 suites, 242 tests, 271 snapshots passed.
• Vite library build; checked the emitted declarations.
• Consumer using the built declarations compiles against React 18 and React 19 types, including a nullable ref shared with a DOM element.
• Descript client React 19 check using these built declarations: the Draft.js ref error is gone; three existing inert errors remain.
• Compared emitted JavaScript before/after for both production files: identical. Live-app validation skipped for this type-only change.
• I used AI.

The fork's plain source typecheck already fails on missing Node ambient types and newer standard-library APIs. With explicit Node types and ES2020 libraries, the React 18 source check passes. The equivalent React 19 source check retains three pre-existing callback-ref return errors in DraftEditorBlock, DraftEditorLeaf, and DraftEditorTextNode; those are outside this PR.

[linear-code]

BIX-8750