Add `get_active_element_css()` to the SeleniumBase API

[mdmintz]

Add get_active_element_css() to the SeleniumBase API

That method currently only exists through js_utils, but it may be useful to have it be directly usable through the main API.
get_active_element_css() returns the CSS Selector for the currently active element.

[mdmintz]

This was resolved in 4.18.2 - https://github.com/seleniumbase/SeleniumBase/releases/tag/v4.18.2