✨ add a GitHub Action for shellcheck'ing on every PR and push to master branch + shellcheck'ed script

shellcheck was complaining:

$ docker run --rm -v "$PWD:/mnt" koalaman/shellcheck:stable -Calways -S warning lsix

In lsix line 196:
    for x in ${!MAPFILE[@]}; do
                 ^------------^ SC2068 (error): Double quote array expansions to avoid re-splitting elements.

In lsix line 208:
        (cd "$arg"; $lsix)
                 ^-------^ SC2164 (warning): Use 'cd ... || exit' or 'cd ... || return' in case cd fails.

Did you mean:
        (cd "$arg" || exit; $lsix)

In lsix line 218:
    resize="[${tilewidth}x${tileheight}]"
    ^----^ SC2034 (warning): resize appears unused. Verify use (or export if used externally).

In lsix line 237:
    while [ $# -gt 0  -a  $# -gt $goal ]; do
                          ^-- SC2166 (warning): Prefer [ p ] && [ q ] as [ p -a q ] is not well defined.

For more information:
  https://www.shellcheck.net/wiki/SC2068 -- Double quote array expansions to ...
  https://www.shellcheck.net/wiki/SC2034 -- resize appears unused. Verify use...
  https://www.shellcheck.net/wiki/SC2164 -- Use 'cd ... || exit' or 'cd ... |...

This PR fixed this and I tested it:

My GitHub Action's Run can be found here.

hackerb9 / lsix

✨ add a GitHub Action for shellcheck'ing on every PR and push to master branch + shellcheck'ed script #75