Fix parsing of nested classes with private fields

The parsing of private fields checks whether or not it is within a
class to determine if it is valid or not. However, the state.inClass
property is incorrect as it marks it as outside a class when the inner
class is closed.

This commit fixes this problem by replacing the state.inClass property
with a class nesting counter.

test/fixtures/experimental/class-private-properties/nested/actual.js

@@ -0,0 +1,40 @@
+class Point {
+    #x = 1;
+    #y = 2;
+
+    constructor(x = 0, y = 0) {
+        #x = +x;
+        #y = +y;
+
+
+        this.foo = class {
+            #x = 1;
+            #y = 2;
+
+            constructor(x = 0, y = 0) {
+                #x = +x;
+                #y = +y;
+            }
+
+            get x() { return #x }
+            set x(value) { #x = +value }
+
+            get y() { return #y }
+            set y(value) { #y = +value }
+
+            equals(p) { return #x === p.#x && #y === p.#y }
+
+            toString() { return `Point<${ #x },${ #y }>` }
+        };
+    }
+
+    get x() { return #x }
+    set x(value) { #x = +value }
+
+    get y() { return #y }
+    set y(value) { #y = +value }
+
+    equals(p) { return #x === p.#x && #y === p.#y }
+
+    toString() { return `Point<${ #x },${ #y }>` }
+}

test/fixtures/experimental/class-private-properties/nested/options.json

@@ -0,0 +1,3 @@
+{
+  "plugins": ["classProperties", "classPrivateProperties"]
+}