Rename all proposal plugins to -proposal- from -transform- (#6570)

packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-spread/expression/actual.js

@@ -0,0 +1 @@
+({ x, ...y, a, ...b, c });

packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-spread/expression/expected.js

@@ -0,0 +1,9 @@
+function _extends() { _extends = Object.assign || function (target) { for (var i = 1; i < arguments.length; i++) { var source = arguments[i]; for (var key in source) { if (Object.prototype.hasOwnProperty.call(source, key)) { target[key] = source[key]; } } } return target; }; return _extends.apply(this, arguments); }
+
+_extends({
+  x
+}, y, {
+  a
+}, b, {
+  c
+});