fix: focus loss inside Shadow DOM in Firefox.

[ivan-sysoi]

Description
This pr fixes shadow dom example in firefox browser.

Issue
Fixes: #5321

Example

Context
ReactEditor.findDocumentOrShadowRoot returns document instead of ShadowRoot in firefox. Because firefox doesn't support getSelection on ShadowRoot.
This breaks the following:

activeElement === el

Here, for example:

slate/packages/slate-react/src/components/editable.tsx

Lines 231 to 241 in 0dc5852

	const root = ReactEditor.findDocumentOrShadowRoot(editor)
	const { activeElement } = root
	const el = ReactEditor.toDOMNode(editor, editor)
	const domSelection = root.getSelection()
	if (activeElement === el) {
	state.latestElement = activeElement
	IS_FOCUSED.set(editor, true)
	} else {
	IS_FOCUSED.delete(editor)
	}

So I decided to drop root.getSelection != null check from ReactEditor.findDocumentOrShadowRoot and added a separate function getSelection with it.

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: 4dde3c6

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

This PR includes changesets to release 1 package

Name	Type
slate-react	Minor

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR