set "thiserror" as a default feature

[PoOnesNerfect]

This change would re-export thiserror::Error as tosserror::Error so that you don't have to depend on thiserror as a dependency anymore.

Example code:

use tosserror::{Error, Toss};

#[derive(Error, Toss, Debug)]
pub enum DataStoreError {
    #[error("invalid value ({value}) encountered")]
    InvalidValue {
      value: i32,
      source: std::num::TryFromIntError,
    },
}