update assignment 4

4-ShellP2/.DS_Store

4-ShellP2/bats/assignment_tests.sh

4-ShellP2/bats/student_tests.sh

4-ShellP2/debug/launch.json

4-ShellP2/debug/tasks.json

4-ShellP2/dragon.c

4-ShellP2/dsh

4-ShellP2/dsh_cli.c

4-ShellP2/dshlib.c

4-ShellP2/dshlib.h

4-ShellP2/gitignore.txt

4-ShellP2/makefile

4-ShellP2/questions.md

4-ShellP2/shell_roadmap.md