Movable props codegen

[KodrAus]

For #360

This PR introduces a new style of props! codegen that will let us move out of individual properties. This not only helps with 2024 edition changes to scoping, it also makes props! much more useful in other contexts, like as evt_props on #[span], because they can now be properly owned.

It's a fairly substantial change. This new method generates a lot more code than the old way, so it'll need some thorough testing to make sure it doesn't bloat compile times too much.

[KodrAus]

The props module could do with some documentation, since it’s the most complex piece of codegen we have.

The book and docs also need updating to make more use of props! now instead of other types now that we can actually use it.