Make version selection smart and flexible

[bewuethr]

Currently, this defaults to v0.8.0 of ShellCheck.

This should be more flexible, and users should be able to select a version, or just use latest (which should be the default).

This could happen in a separate action (something like setup-shellcheck), which checks what version the runner image has (Ubuntu 20.04 has an older version, 22.04 has the latest), what the latest version is, and potentially fetches (and caches) such a version. The call to that action can be added as an additional step in the shellcheck-action composite action.