Replace defaultOptions with addOptions

[philippkuehn]

This PR changes how default options are defined in extensions.

This has the following advantages:

• more consistent naming (like addStorage, addInputRules, addProseMirrorPlugins, …)
• when extending you can use this.parent within addOptions like everywhere else
• within addOptions() you have access to this.name which is useful in some situations

Since this is a breaking change, a warning is thrown if you keep using defaultOptions.
But it will remain backwards compatible for a while.

Before:

// define extension
const extension = Extension.create({
  defaultOptions: {
    awesomeness: 100,
  },
})

// extend options
const extendedExtension = extension.extend({
  defaultOptions: {
    ...extension.options,
    epicness: 100,
  },
})

After:

// define extension
const extension = Extension.create({
  addOptions() {
    return {
      awesomeness: 100,
    }
  },
})

// extend options
const extendedExtension = extension.extend({
  addOptions() {
    return {
      ...this.parent?.(),
      epicness: 100,
    }
  },
})