fix: 🐛 export MARKS and EMPTY_DOCUMENT as ESM friendly

[mika-f]

MARKS and EMPTY_DOCUMENT are not exports in a format that supports in ESM, so using them in a project with type: module will result in an error like the one below:

error Named export 'MARKS' not found. The requested module '@contentful/rich-text-types' is a CommonJS module, which may not support all module.exports as named exports.

This PR fixes modules that are export default and default as XXX to named exports, allowing ESN project to build.
This may solve issue #395.