Added an example for IndexedStack widget (#105318)

* Added an example for IndexedStack

* Added tests for the IndexedStack example

* Fixed type issue for onSubmitted callback functions

* Fixed documentation and moved files to their appropriate places

* Fixed documentation and moved files to their appropriate places

* Moved test files to their appropriate places

* Moved test files to their appropriate places

* Fixed file path in documentation

* Remove trailing space

* Formatting changes

* Remove extra line

* Further formatting changes

* Further formatting changes

* fix comma and inline

Co-authored-by: Greg Spencer <gspencergoog@users.noreply.github.com>

* Formatting

* indentation and formatting

* Formatting

* Formatting

* Formatting

* Removed duplicate chevron

* better wording on documentation

Co-authored-by: Tong Mu <dkwingsmt@users.noreply.github.com>

* Added testing for state preservation

Co-authored-by: Greg Spencer <gspencergoog@users.noreply.github.com>
Co-authored-by: Tong Mu <dkwingsmt@users.noreply.github.com>