feat: batch of new functions (slices, regexp, conversion)

[42atomys]

Description

This pull request add a batch of new functions according to requests on sprig issues and pull requests or direct on sprout.

Changes

• Add toLocalDate function to conversion registry
• Add regexpFindGroups, regexpAllGroups, regexpFindNamed, regexpAllNamed functions to regex registry
• Add flatten, flattenDepth to slices registry

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.
• [x] I have updated the documentation accordingly.
• [x] This change requires a change to the documentation on the website.

[codecov[bot]]

Codecov Report

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

Files with missing lines	Coverage Δ
registry/conversion/conversion.go	92.3% <100.0%> (+0.3%)	:arrow_up:
registry/conversion/functions.go	100.0% <100.0%> (ø)
registry/regexp/functions.go	100.0% <100.0%> (ø)
registry/regexp/regexp.go	94.4% <100.0%> (+0.6%)	:arrow_up:
registry/slices/functions.go	100.0% <100.0%> (ø)
registry/slices/helpers.go	100.0% <100.0%> (ø)
registry/slices/slices.go	96.7% <100.0%> (+0.1%)	:arrow_up: