Front-end has implemented hard-coded `NodeType` and `MarkType` names instead instead of `NodeType.HEADING`.

[stefrado]

Description:

The current implementation hard-coded the NodeType and MarkType. This caused #21 to need a front-end change.
The expected behavior is that we use NodeType.HEADING for example instead of hard-coding "heading".

With this hard-coded string it is hard to make sure that change the name of the node/mark without making breaking changes.

Attention points

These changes are most likely required in the test files and plugins.