Make/data structure/linked list.cpp generic

[AmirCheshmberah]

Description of Change

Make ./data structure/linked_list.cpp class generic
remove isDigit() function because class in generic now
change t variable to temp for better reading and prevent confusing with typename T
add braces for switch-case to prevent crosses initialization error

Checklist

• Added description of change
• Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one, as yours may be a duplicate.
• I acknowledge that all my contributions will be made under the project's license.

Notes:
there is reverse() function declaration on line 96 and no implementation