Add additionalFields as option with ($page)=>{} functions

[b0g3r]

For example, we want to add color as a field to a tree.

In the old times, we would add color to a plugin source code: color: page.frontmatter.color || 'blue'. My proposal is to add an option additionalFields to plugin interface:

[
  '@b0g3r/generate-tree',
  {
    locales: LOCALES,
    additionalFields: {
      color: (page) => page.frontmatter.color || 'blue',
      title: (page) => page.frontmatter.title || page.title || '',
      ...
    }
  },
],

We should carefully check the ability to use these dynamic field names in the plugin type system.