string_extension library Null safety

1. headless_components
2. string_extension library

headless_components package

1. Libraries
2. bool_extension
3. datetime_extension
4. extensions
5. inactivity_detector
6. int_extension
7. iterable_extension
8. list_extension
9. map_extension
10. num_extension
11. object_extension
12. pair
13. string_extension