Introduce JetModifierKeywordToken to mark keywords which can be used as modifiers

Change JetModifierList and JetModifierListOwner api to accept only those tokens
Make JetCompletionMultiTestBase extend JetFixtureCompletionBaseTestCase
Use common infrastructure for JetMultifileBasicCompletionTest
Switch JetMultifileBasicCompletionTest to generated approach
Buildserver auto merge