refactor: update isEditable implementation

[ap0nia]

Summary

Improve the utility of the isEditable state stored in context. It is writable, and both subscribes to the editor's editable state, as well as writes to it when it is set.

Next Steps

Alternatively, maybe no setIsEditable function needs to be made at all; it can be done in the same function as setEditor since that already receives the editor.