[Refactoring] Tweak token consumers

parseImport should return parsed `Import`s.
Add test for ddoc unittest feature (1)
Add test for ddoc unittest feature (2)
fix Issue 11677 - user defined attributes must be first

Parse prefix attributes in while loop of parseDeclDefs
fix Issue 11678 - user defined attributes cannot appear as postfixes
fix Issue 11679 - user defined attributes not allowed for local auto declarations
fix Issue 11680 - user defined attributes for type inference