fix: remove Default impl for LeptosOptions and ConfFile

[chrisp60]

Closes #3268

Originally discussed adding a macro_rules! for this. Maybe it is best if we do not add more public API for now.

This is what the end user needs to write.

LeptosOptions::builder().output_name(env!("CARGO_CRATE_NAME")).build()

After thinking it over, this does not feel like enough of an inconvenience to bring in a #[macro_export].

If anyone feels otherwise, we can always add it in later.