🐍 Let serde be an optional dependency

[tosti007]

This PR marks serde as an option dependency and only derives the various Serialize and Deserialize traits when this feature is enabled.

Regardless of not forcing the user to have the serde dependency, having the serde feature of egui (optionally) enabled is a requirement for #12.

[tosti007]

Turns out the CI compiles with --no-default-features. However the examples do some save and load the previous app state, which requires the serde feature. As I see it we have two options here:

1. Add the serde dependency for the examples.
2. Add some #[cfg(feature = "serde")] and #[allow(clippy::unused_mut)] statements.

I am not sure what the preferred route is here, if option 1 is the desired solution, I am not sure how to do such change.

[tosti007]

Just rebased on latest main and I think no additional changes were needed.