feat: add option to customize row selection and row collapse

[lukasnys]

With the current version of ember-table, you can only customize the row selection and row collapse visualisation through CSS overrides of the input type="checkbox" element and its empty sibling span element.

This PR aims to unlock a way of customizing the row selection and row collapse visualisation as a whole, allowing people to use their own (design system) components instead of using CSS.

[lukasnys]

Need to figure out why the tests are failing. Named blocks are only supported as of Ember 3.25+ but the ember-named-blocks-polyfill package should support it for versions before that 🤔