Add more special methods to Namespace for ease of use.

* __getattr__ would allow it to no longer AttributeError while
  simultaneously letting linters and type checkers understand that
  this is a bag of dynamic attributes.
* __contains__ allows checking for the existence of an attribute
* __getitem__ allows accessing a value even if its name is not a valid
  Python identifier.

discord/app_commands/namespace.py

@@ -68,7 +68,8 @@ class Namespace:
 
     This class is deliberately simple and just holds the option name and resolved value as a simple
     key-pair mapping. These attributes can be accessed using dot notation. For example, an option
-    with the name of ``example`` can be accessed using ``ns.example``.
+    with the name of ``example`` can be accessed using ``ns.example``. If an attribute is not found,
+    then ``None`` is returned rather than an attribute error.
 
     .. versionadded:: 2.0
 
@@ -80,6 +81,13 @@ class Namespace:
         .. describe:: x != y
 
             Checks if two namespaces are not equal.
+        .. describe:: x[key]
+
+            Returns an attribute if it is found, otherwise raises
+            a :exc:`KeyError`.
+        .. describe:: key in x
+
+            Checks if the attribute is in the namespace.
 
     This namespace object converts resolved objects into their appropriate form depending on their
     type. Consult the table below for conversion information.
@@ -213,6 +221,15 @@ class Namespace:
             return self.__dict__ == other.__dict__
         return NotImplemented
 
+    def __getitem__(self, key: str) -> Any:
+        return self.__dict__[key]
+
+    def __contains__(self, key: str) -> Any:
+        return key in self.__dict__
+
+    def __getattr__(self, attr: str) -> Any:
+        return None
+
     def _update_with_defaults(self, defaults: Iterable[Tuple[str, Any]]) -> None:
         for key, value in defaults:
             self.__dict__.setdefault(key, value)