Psi: Introduce KtScriptInitializer, a separate entity for script initializers which are quite a bit different from class initializers

compiler/testData/psi/annotation/ShortAnnotations.txt

@@ -613,7 +613,7 @@ JetFile: ShortAnnotations.kt
       PsiErrorElement:Expecting member declaration
         PsiElement(IDENTIFIER)('zoo')
       PsiWhiteSpace(' ')
-      ANONYMOUS_INITIALIZER
+      CLASS_INITIALIZER
         PsiElement(init)('init')
         PsiWhiteSpace(' ')
         BLOCK

compiler/testData/psi/annotation/at/validDeclarations.txt

@@ -435,7 +435,7 @@ JetFile: validDeclarations.kt
           INTEGER_CONSTANT
             PsiElement(INTEGER_LITERAL)('1')
       PsiWhiteSpace('\n\n    ')
-      ANONYMOUS_INITIALIZER
+      CLASS_INITIALIZER
         MODIFIER_LIST
           ANNOTATION_ENTRY
             PsiElement(AT)('@')