feat(types): add properties-of<T> type

docs/annotating_code/type_syntax/atomic_types.md

@@ -52,6 +52,7 @@ Atomic types are the basic building block of all type information used in Psalm.
 - `key-of<Foo\Bar::ARRAY_CONST>`
 - `value-of<Foo\Bar::ARRAY_CONST>`
 - `T[K]`
+- [`properties-of<T>`](utility_types.md#properties-oft)
 
 ## Top types, bottom types
 

docs/annotating_code/type_syntax/utility_types.md

@@ -0,0 +1,40 @@
+# Utility types
+
+Psalm supports some _magical_ utility types that brings superpower to the PHP type system.
+
+## `properties-of<T>`
+
+This collection of _utility types_ construct a keyed-array, with the names of non-static properties of a class as keys,
+and their respective types as values. This can be useful if you need to convert objects into arrays.
+
+```php
+class A {
+  public string $foo;
+  public int $bar;
+
+  /**
+   * @return properties-of<self>
+   */
+  public function asArray(): array {
+    return [
+      'foo' => $this->foo,
+      'bar' => $this->bar,
+    ];
+  }
+
+  /**
+   * @return list<key-of<properties-of<self>>>
+   */
+  public function attributeNames(): array {
+    return ['foo', 'bar']
+  }
+}
+```
+
+### Variants
+
+Note that `properties-of<T>` will return **all non-static** properties. There are the following subtypes to pick only
+properties with a certain visibility:
+- `public-properties-of<T>`
+- `protected-properties-of<T>`
+- `private-properties-of<T>`