add class SLL
pass test 1
add test
Add Node class
- add size variable and header node
- create constructor for init SLL
implement add all objects to list
add test insert
add get node by index
pass insert
refactor
add delete test
pass delete
Add test first
pass test first
add test last
pass test last
add test before
pass test before
add test after
pass test after
add test find
pass find node by object