Add Cramer's rule for solving system of linear equations in two variables (#7547)

* added script for solving system of linear equations in two variables

* implemented all the suggested changes

* changed RuntimeError to ValueError

* Update matrix/system_of_linear_equation_in_2_variables.py

* Update matrix/system_of_linear_equation_in_2_variables.py

* Update and rename system_of_linear_equation_in_2_variables.py to cramers_rule_2x2.py

Co-authored-by: Christian Clauss <[email protected]>