feat: introduce toduration conversion function

[42atomys]

Description

This pull request introduce the toDuration as proposed in #8 and https://github.com/Masterminds/sprig/issues/388 using the cast package.

Changes

• Add the function toDuration

Fixes #8

Checklist

• [x] I have read the CONTRIBUTING.md document.
• [x] My code follows the code style of this project.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.
• [ ] I have updated the documentation accordingly.
• [x] This change requires a change to the documentation on the website.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.7%. Comparing base (f253487) to head (75537b4).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #27 +/- ## ===================================== Coverage 99.7% 99.7% ===================================== Files 13 13 Lines 1398 1401 +3 ===================================== + Hits 1394 1397 +3 Misses 2 2 Partials 2 2 ```

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