Squashed commits (initially a commit from rokups + many rework by ocornut. keeping them separate commits made rebasing unnecessarily tricking so merged from 2024/02)
ImStrv: many fixes (see details), added imconfig class extension example, added natvis description.
ImStrv: rework toward ensuring End is always set to constant can be compile time calculated
ImStrv: using length(), fix ambiguous empty() function, fix altered behaviors, removed unused operators.
ImStrv: various tweaks and fixes. removed ImGuiTextRange from ImGuiTextFilter, fix test engine hooks, removed constructor only used twice.
