feat: merge context settings, replace if_filled tag with var

[JuroOravec]

This MR:

• Removes the nodes and template tags involved in the If/Elif/Else logic ({% if_filled %}, etc). Instead, the context is populated with a slots dict that tracks which slots have been filled, and can be used like {% if component_vars.is_filled.<slot_name> %}

• Merges context_behavior and slot_context_behavior into context_behavior. It now has two options - "isolated" and "django" (this was previously called global). isolated is the default.

• Revamps the slot resolution logic. It has now been moved "upstream", meaning that the slot resolution is now done in Component.render(). And so, by the time we reach SlotNode.render(), we only need to look up the corresponding slot fill.

• To make it easier to onboard people to the slot resolution flow, I've documented the flow in docs/slot_rendering.md

• Updates README

• See other comments for smaller changes

This is a big MR, but also about 2/3 of it are documentation and test changes.

Closes https://github.com/EmilStenstrom/django-components/issues/280 Closes https://github.com/EmilStenstrom/django-components/issues/450

[dylanjcastillo]

This looks great. And the new documentation section about slot rendering is super useful. Thank you, Juro!

The only thing I noticed is that there were some small formatting changes in the README. I wonder if we should use a pre-commit hook for md files? Because whenever I change the README, the same thing happens.

[JuroOravec]

@dylanjcastillo Thanks!

On README formatting - Yeah I think I would be good to have README linted too, do you know how to set it up?

[dylanjcastillo]

I’ll give it a go in the coming days!