add ability to point to external builtin registry

[neerfri]

Summary

This PR adds the ability to add a custom builtin registery

Changes

• Add option to JushBash.new/1: custom_builtin_registery
• The interface for methods inJustBash.Commands.Registry get's the JustBash instance as first argument.
• Added the ability in JustBash.Interpreter.Executor.execute_builtin/4 to execute a function-defined builtins

Related Issues

Non.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Testing

Describe how this was tested:

• Added new tests
• All existing tests pass
• Tested manually

Checklist

• My code follows the style guidelines of this project
• I have run mix format
• I have run mix credo and addressed any issues
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes
• I have updated the documentation accordingly
• I have updated the CHANGELOG.md (for non-trivial changes)

[neerfri]

Hi Team! Awesome project!
Any interest in this addition?

@davydog187 @ivarvong @cjbell