Generic return type for Node.fragment

[12joan]

Description
Use generics for the return type of Node.fragment so that it matches the children of the root you pass in.

Example

interface ParagraphElement extends BaseElement {
  type: 'paragraph'
  children: InlineNode[]
}

// Type of fragment is InlineNode[]
const fragment = Node.fragment(paragraph, range)

Checks

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn fix.)
• The relevant examples still work. (Run examples with yarn start.)
• You've added a changeset if changing functionality. (Add one with yarn changeset add.)

[changeset-bot]

🦋 Changeset detected

Latest commit: b7307ff

The changes in this PR will be included in the next version bump.

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