Update decorators parsing (#7719)

* Update decorators parsing

This commit introduces three changes:
1) Class properties can be decorated
2) Decorators can contain arbitrary expressions, using @(...)
3) The Decorator node type has a new property, "arguments". This
    makes it possible do distinguish @dec() and @(dec()), which have
    different behaviors because @(dec()) is equivalent to @(dec())().

* Rename Decorator#expression to Decorator#callee

* Add test for @dec()()

packages/babel-types/src/definitions/experimental.js

@@ -135,11 +135,18 @@ defineType("Import", {
 });
 
 defineType("Decorator", {
-  visitor: ["expression"],
+  visitor: ["callee", "arguments"],
   fields: {
-    expression: {
+    callee: {
       validate: assertNodeType("Expression"),
     },
+    arguments: {
+      optional: true,
+      validate: chain(
+        assertValueType("array"),
+        assertEach(assertNodeType("Expression", "SpreadElement")),
+      ),
+    },
   },
 });