Support overriding Jinja tags / functions / filters

Is your proposal related to a problem?

Part of Jinja2 support plan #180. Version 1.5.0 ships with experimental Jinja support, missing the ability to override tags / functions / filters in particular.

Describe the solution you'd like

A setup similar to how Django Templates tags overrides work, with support for declaring overrides via YAML, or default values globally in Python. In Jinja it would make sense to also do this for functions, not just tags. And possibly filters while we’re at it (see also #114).

Describe alternatives you've considered

N/A

Additional context

I suspect this will require messing with the environment where we inject the custom context: https://github.com/torchbox/django-pattern-library/blob/main/pattern_library/loader_tags.py#L199. However I don’t have a good enough understanding of Jinja to know for sure.

Start in pattern_library/loader_tags.py around line 199 and trace how the Jinja environment receives custom context. Compare this with the existing Django template tag override behavior, then determine how YAML or global Python declarations should cover Jinja tags, functions, and filters; done means each requested override type can be declared and applied.