Implement std::error::Error for TemporalError (#664)

Currently, `TemporalError` does not implement `std::error::Error`, which
makes `TemporalResult` incompatible with ecosystem crates for error
conversions, such as `anyhow`.

The fix is a simple one-line addition to implement the error trait,
since `TemporalError` already implements `Display` and `Debug`.

---------

Co-authored-by: Leandro Vandari <leandrovandari@gmail.com>

Author: LeandroVandari

src/error.rs

@@ -135,6 +135,8 @@ impl TemporalError {
     }
 }
 
+impl core::error::Error for TemporalError {}
+
 impl fmt::Display for TemporalError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "{}", self.kind)?;