Add support Class field declarations

Author: shenlibo

test/compare-acorn-es2019/class-field-declarations.expected.js

@@ -0,0 +1,9 @@
+class FieldDeclarations{
+    a = 1
+    b = function () {}
+}
+class PrivateFields{
+    #a = 1
+    #b = function () {}
+}
+