Move extended property comments to create table comment
There is no need to change from extended props to plain comments. Since I figured out that the descriptions are displayed in the visual designer part of the table in the SSDT project's tables, this is not really required to change. 

This article by Jamie Thomson explains the details and that the descriptions of the columns are getting visible there as soon the column "Description" in the designer is enabled. Also, they are automatically created into the create script if entered in the designer what is pretty cool.