Use `findDiffStart` instead of `equalityDeep`

[BrianHung]

Use ProseMirror method findDiffStart to compare / diff documents instead of converting to JSON and then using lib0's equalityDeep:

- !fun.equalityDeep(view.state.doc.type.createAndFill().content.toJSON(), view.state.doc.content.toJSON())
+ view.state.doc.content.findDiffStart(view.state.doc.type.createAndFill().content) != null

Reasoning is equalityDeep fails on two equal JSON objects created by ProseMirror's toJSON(), because ProseMirror uses Object.empty(null) to initialize node attrs, which results in equalityDeep returning false because Object.create(null).constructor is undefined (instead of Object as expected). See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/create#custom_and_null_objects.

Another work around would be to use

!fun.equalityDeep(
  JSON.parse(JSON.stringify(view.state.doc.type.createAndFill().content.toJSON())), 
  JSON.parse(JSON.stringify(view.state.doc.content.toJSON())),
);

which works correctly by converting Object.create(null) and forcing it to inherit the Object.constructor method. But

1. we're already working with ProseMirror objects, and
2. it requires an extra step in JSON.parse(JSON.stringify(.

[dmonad]

Thanks! @BrianHung Merged and published the fix in v1.0.9