Relevant Linear issue [[ https://linear.app/comm/issue/ENG-1296/add-shellcheck-or-something-similar-to-the-ci | here ]].
This diff adds [[ https://www.shellcheck.net/ | ShellCheck ]] to the CI, a static analysis tool for shell scripts that provides warnings and errors.
We install `ShellCheck` and also set `globstar` so we can use `**` to recursively match all `.sh` files in the repository. These files are then evaluated by `ShellCheck`.