Extend createNormalizeTypesPlugin with strictNotType

[ixahmedxi]

Description: Forcing the first block of the editor to be an h1 still makes it possible to move that block down the editor while replicating h1 blocks along the way. I have detailed the bug completely in this codesandbox: https://codesandbox.io/s/suspicious-wave-6g1zs?file=/src/Editor.tsx

[ixahmedxi]

I have resulted in my case to making a new custom forced layout plugin that enforces the first block to be an h1 and the second block to not be an h1 but I think the idea of having a strictNotType or something along these lines in the rules for the createNormalizeTypesPlugin would be very beneficial to support more use cases.