I'm currently working on this(Add support for CHECK constraints), as a part of my GSOC project.

As Isaac has written in his comment, this feature involves three parts. The order in which I want to work on this is the following - 

2. Update parser to understand this syntax

3. Update the export interface to allow including these in an export

For the first part-

I am thinking of implementing this something like query by example, 

  - we show the operator in a select dropdown, and 2 options, to enter value or another column name

The constraints will be shown like indexes are, in the table structure, where there will be an option to delete or add new check constraint on the table.

For second and third parts-

Not sure yet.