Port bevy_yarnspinner to Bevy 0.19

[uros117]

Summary

• bump the Bevy-facing Yarn Spinner crates from Bevy 0.18 to 0.19
• adapt SystemState::get_mut calls to the new fallible API
• update mutable asset access and untyped asset loading for Bevy 0.19
• replace deprecated chained system conditions with and_then
• migrate the Bevy plugin test utilities to the updated APIs

Testing

• cargo check -p bevy_yarnspinner
• cargo test -p bevy_yarnspinner

The plugin suite passes with 67 unit/integration tests and 18 doctests; 2 pre-existing tests remain ignored.

[janhohenheim]

Thanks! Updated some missing stuff :)