Export createColGroup utility function

[fujita-h]

Please describe your changes

Changed to export the createColGroup function added in #4281.

How did you accomplish your changes

The createColGroup function is needed when extending the output with renderHTML() in extend(), so it should be exported.

How have you tested your changes

n/a

How can we verify your changes

Functions can be imported successfully using

import { createColGroup } from @tiptap/extension-table

Remarks

n/a

Checklist

• The changes are not breaking the editor
• Added tests where possible
• Followed the guidelines
• Fixed linting issues

Related issues

#4280, #4281

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[bdbch]

LGTM