linting: add shelcheck for shell script lint

[L-series]

Add linting for shell scripts in the scipts directory as well as any *.sh file accross the project using shellcheck.

[L-series]

@hanno-becker Hello, please see my comment above regarding what the issue was with the shfmt syntax. I've changed that syntax and I simplified the logic behind the shellcheck implementation.

I'm adding a variable which tracks the shells scripts which exist across the project and I pass that to both shfmt and shellcheck instead of recomputing it for shellcheck.