[decimal] Implement root() function to calculate the n-th root of a Decimal value (#40)

This pull request introduces the `root` function to the DeciMojo
library, which calculates the n-th root of a Decimal value using the
Newton-Raphson method. The most important changes include adding the
`root` function, updating the `Decimal` struct to include the `root`
method, and adding comprehensive tests for the new functionality.

### New Functionality:
* `src/decimojo/exponential.mojo`: Added the `root` function to
calculate the n-th root of a Decimal value. This function includes
special cases for handling different values of `n` and `x`, and uses the
Newton-Raphson method for iteration.

### Updates to Existing Code:
* `src/decimojo/decimal.mojo`: Updated the `Decimal` struct to include
the `root` method, which calls the new `root` function.
* `src/decimojo/__init__.mojo`: Updated the import statements to include
the new `root` function from the `exponential` module.

### Testing:
* `tests/test_root.mojo`: Added comprehensive tests for the `root`
function, covering basic root calculations, fractional inputs, edge
cases, error conditions, precision, and mathematical identities.

### Configuration:
* `mojoproject.toml`: Added commands for testing and benchmarking the
new `root` function.

Author: forfudan