This commit makes the magic number in the trix file format configurable.
- The `Encode` and `Decode` functions in the `trix` package now accept a `magicNumber` string as a parameter.
- The example in `examples/main.go` has been updated to demonstrate the use of the new configurable magic number.
- The tests for the `trix` package have been updated to verify the new functionality, including tests for invalid magic numbers.
