fix: weak guard on DataTransfer to not rely on current window

[anho]

Description
As previously asked in Slack, this helps in automated testing where it's currently not possible to properly transfer data between the host app and the application under test. The previous instanceof window.DataTransfer fails as Cypress is not able to properly get hold of the window object Slate is referring to (despite using cy.window). To mitigate that the check is now checking for the constructor name.

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: d7834fd

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	Patch

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