Implement parsing annotations and modifiers starting with '@'

compiler/testData/psi/annotation/at/primaryConstructor.kt

@@ -0,0 +1,9 @@
+class A1 @Ann1("") ()
+
+class A2 @Ann2("")(x: Int) : B {
+}
+
+class A3 [Ann3] private @(x: Int)
+class A4 [Ann4] @private @(x: Int)
+class A5 private @Ann4(x: Int) : B
+class A6 [Ann5] @private @ [Ann6]()