Scrollbar.js Sanitizations

Background: First Pr #33245 was to just decoupling scrollbar functionality form modal.js , coping the existing code as it was.

This Pr optimizes functionality:

• Removes .modal-open class as it is already handled by scrollbar.js
• _modal.scss make modal overflowY: auto by default (was on ``.modal-open`, which is the same)
• scrollbar.js: keep initial body overflowY value as attribute and re-set it properly & test
• scrollbar.js: avoid to create data-bs attributes for null values
• Revisit modal, offcanvas & scrollbar tests
• set the base to support different rootElement (than body) helping #33018 an #33574