fix: Make quote blocks isolating

[matthewlipski]

Summary

This PR makes quote blocks isolating. That means when pasting content into an empty quote block, it won't overwrite the block type to that of the clipboard content.

Closes #2236

Rationale

There is a scenario where the user wants to paste some text for a quote, so they create a new quote block using the slash menu. They then paste the quoted text into the newly created block, and the type changes to something else. This is not great for UX.

Changes

• Added isolating flag to quote block.

Impact

N/A

Testing

We already have tests for pasting in custom blocks which are isolating.

Screenshots/Video

N/A

Checklist

• [X] Code follows the project's coding standards.
• [X] Unit tests covering the new feature have been added.
• [X] All existing tests pass.
• [X] The documentation has been updated to reflect the new feature

Additional Notes