Add @ludeeus ShellCheck GitHub Action

[atrepca]

Reopened #6 with adding a GitHub action for ShellCheck.

Tested this with my fork and it works as expected:

• Opened PR using the current main: Test ShellCheck action atrepca/hass-addons#3
• This triggered the GitHub Action which failed as expected with the errors:

./amazon-s3-backup/run.sh:17:8: warning: Declare and assign separately to avoid masking return values. [SC2155]
./amazon-s3-backup/run.sh:18:8: warning: Declare and assign separately to avoid masking return values. [SC2155]

I excluded SC1008, the with-contenv shebang is appropriate for Docker even though ShellCheck doesn't like it.

[atrepca]

Following up here @thomasfr, any chance this can get a review? Thanks,