Add a template filter.

        This is designed to be used on the application directly. An
        example usage,

        .. code-block:: python

            def to_upper(value):
                return value.upper()

            app.add_template_filter(to_upper)

        Arguments:
            func: The function that is the filter.
            name: The filter name (defaults to function name).

def add_template_filter(self, func: Callable, name: Optional[str]=None) -> None:
        """Add a template filter.

        This is designed to be used on the application directly. An
        example usage,

        .. code-block:: python

            def to_upper(value):
                return value.upper()

            app.add_template_filter(to_upper)

        Arguments:
            func: The function that is the filter.
            name: The filter name (defaults to function name).
        """
        self.jinja_env.filters[name or func.__name__] = func