Stubs for import list
FIXUP LATER: Base stubs for import directive
Stubs: get type parameter list by stub
PSI: get import list and import directive by stubs
Stubs for JetModifierList base
PSI: use stubs to get modifier list
BASE: Stubs for type constraint list
PSI: get type constraint list by stub
Minor: make JetTypeElement an interface
REWORD: Stubs: reduce the amount of boilerplate classes

Extract a common type for placeholder stub elements