Preserve block identity when editing from the block's start #550

Describe the bug
At Mintter block Identity is really important. Specially because we let users create references to content inside blocks using its ID. I Notice that when I press Enter or Backspace in front of any block with content. I understand block identity might not be relevant to most use cases, but I find this could be a problem when you rely on content being accessible by a particular block ID.

To Reproduce

1. write some content in a block and remember the block id
2. move the cursor to the beginning of the block
3. press Enter or Backspace (backspace if you are not on the first block of the document or of a nested list)
4. see the blockID changed to the current block content.

Here's a video showing how the ID of the block changes when I press Enter in front of a block

And here's what happens when pressing Backspace in front of a block

How to solve this

I did some work to solve this on our side, and if this is relevant to the library I can share the solution with it.