feat: turn into multiple lines

[LucasXu0]

Feature Preview

showcase.mp4

• select multiple lines with block selection style.
• turn into multiple lines
• a. take care of the blocks that are not allowed to have children, such as headings, quotes, and callouts, when turning them into...
  • nested list -> heading
  • nested list -> callout
  • nested list -> quote
• b. Only transform nodes at the same level into another block type (not including their children, if they have any)
  • multiple nested list -> another block type
• c. revert back
  • block -> block -> undo -> redo
  • nested list -> block -> undo -> redo
  • nested list with children - > block -> undo -> redo
• known issues
  • update selection after turning block into another block
  • sometimes, block selection doesn't work
• unit tests
  • a.
  • b.
  • c.
• integration test
  • turn into multi lines

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PR Checklist

• My code adheres to AppFlowy's Conventions
• I've listed at least one issue that this PR fixes in the description above.
• I've added a test(s) to validate changes in this PR, or this PR only contains semantic changes.
• All existing tests are passing.