Allow to separate top-level script declarations and statements with semicolon

This fix allows to write one-liner scripts like "val x = 3; println(x)", which
were previously rejected by the parser.

compiler/testData/psi/script/OneLiner.kts

@@ -0,0 +1 @@
+val x = 6; println(x * 7)

compiler/testData/psi/script/OneLiner.txt

@@ -0,0 +1,35 @@
+KtFile: OneLiner.kts
+  PACKAGE_DIRECTIVE
+    <empty list>
+  IMPORT_LIST
+    <empty list>
+  SCRIPT
+    BLOCK
+      PROPERTY
+        PsiElement(val)('val')
+        PsiWhiteSpace(' ')
+        PsiElement(IDENTIFIER)('x')
+        PsiWhiteSpace(' ')
+        PsiElement(EQ)('=')
+        PsiWhiteSpace(' ')
+        INTEGER_CONSTANT
+          PsiElement(INTEGER_LITERAL)('6')
+        PsiElement(SEMICOLON)(';')
+      PsiWhiteSpace(' ')
+      SCRIPT_INITIALIZER
+        CALL_EXPRESSION
+          REFERENCE_EXPRESSION
+            PsiElement(IDENTIFIER)('println')
+          VALUE_ARGUMENT_LIST
+            PsiElement(LPAR)('(')
+            VALUE_ARGUMENT
+              BINARY_EXPRESSION
+                REFERENCE_EXPRESSION
+                  PsiElement(IDENTIFIER)('x')
+                PsiWhiteSpace(' ')
+                OPERATION_REFERENCE
+                  PsiElement(MUL)('*')
+                PsiWhiteSpace(' ')
+                INTEGER_CONSTANT
+                  PsiElement(INTEGER_LITERAL)('7')
+            PsiElement(RPAR)(')')