Automatically identify the class name based on the specified line number. (#2280)

* identify the scene name based on the line number

* resolving a minor bug in string_mobject

* removing bug of string validation

* Update manimlib/default_config.yml

Co-authored-by: Splines <37160523+Splines@users.noreply.github.com>

* Update manimlib/extract_scene.py

Co-authored-by: Splines <37160523+Splines@users.noreply.github.com>

* update search scene names

---------

Co-authored-by: Splines <37160523+Splines@users.noreply.github.com>