Completely rewritten keyword completion implementation using another approach

#KT-2816 Fixed

idea/testData/completion/keywords/InString.kt

@@ -4,8 +4,4 @@ class TestClass {
     }
 }
 
-// ABSENT: abstract, annotation, as, break, by, catch, class, continue, default, do
-// ABSENT: else, enum, false, final, finally, for, fun, get, if, import, in
-// ABSENT: internal, is, null, object, open, out, override, package, private, protected, public
-// ABSENT: ref, return, set, super, This, this, throw, trait, true, try, type, val, var
-// ABSENT: vararg, when, where, while
+// NUMBER: 0