Experimental code path for main window rework (unified webviews)

I've started porting/enhancing the deck browser to Svelte yesterday with the intention to unify the three separate webviews of the main window for a more modern, responsive experience.

@dae, you said:
>Updating the deck list and reviewer screens separately would be logical steps towards that goal, and if it’s done by creating a new code path that can be opted into with a feature flag, then it could be started right now.

I'd like to create such an optional code path for this project. Adding a feature flag that's toggleable in the preferences - analogous to the one for the v3 scheduler - was straightforward, but at
https://github.com/ankitects/anki/blob/95dbf30fb97ac5f1e3f3c0118e4bc2137d1717bf/qt/aqt/main.py#L873
the collection isn't loaded yet, so I can't call `self.col.get_config_bool`. Is there another way to store/retrieve such a flag?

---
Also, what would be a good name for the feature flag?

Some ideas:
- `UNIFIED_MAIN_WINDOW`
- `V3_MAIN_WINDOW`
- `EXPERIMENTAL_MW`