Add support for Readonly mode

[VanishMax]

New feature was introduced in the EditorJS v2.19.0: readonly mode. Here's the description: https://editorjs.io/configuration#read-only-mode

For the support, the's a need to add the property to the block class:

static get isReadOnlySupported () {
  return true;
}

[VanishMax]

Implemented in #5

[maxstue]

@arwazjamal Please add this support, because this is a way better table implementation than the official one :)