Private class methods stage 3 (#8654)

* Add private method syntax support

* Add private method spec support

* Add private method loose support

* Throw error if static private method is used

* Add more isStatic & isMethod checks

* Remove `writable:false` from private method inits

`writable` is false by default.

* Add private method func obj equality check

* Throw if private accessor is used

* Add check for fields === private method loose mode

* Throw buildCodeFrameErrors instead of Errors

* Move obj destructuring inside for loop

* Remove "computed" from ClassPrivateMethod type def

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method/assignment/exec.js

@@ -0,0 +1,11 @@
+class Foo {
+    constructor() {
+      this.publicField = this.#privateMethod();
+    }
+  
+    #privateMethod() {
+      return 42;
+    }
+  }
+  
+  expect((new Foo).publicField).toEqual(42);

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method/assignment/input.js

@@ -0,0 +1,9 @@
+class Foo {
+  constructor() {
+    this.publicField = this.#privateMethod();
+  }
+
+  #privateMethod() {
+    return 42;
+  }
+}

packages/babel-plugin-proposal-private-methods/test/fixtures/private-method/assignment/output.js

@@ -0,0 +1,15 @@
+var Foo = function Foo() {
+  "use strict";
+
+  babelHelpers.classCallCheck(this, Foo);
+
+  _privateMethod.add(this);
+
+  this.publicField = babelHelpers.classPrivateMethodGet(this, _privateMethod, _privateMethod2).call(this);
+};
+
+var _privateMethod = new WeakSet();
+
+var _privateMethod2 = function _privateMethod2() {
+  return 42;
+};