Add CI workflow to check for problems with shell scripts

[MatteoPologruto]

On every push or pull request that modifies one of the shell scripts in the repository, and periodically, the workflow:

• Runs ShellCheck to detect common problems.
• Runs shfmt to check formatting.
• Checks for forgotten executable script file permissions.

Scripts inside subdirectories of deps have been excluded from the checks. Most of the suggestions made by ShellCheck have been applied to build_libs.sh, except for this one, since it should not be an error:

In ./deps/build_libs.sh line 3:
export PREFIX=/opt/lib/${CROSS_COMPILE}
                       ^--------------^ SC2[15](https://github.com/MatteoPologruto/crossbuild/actions/runs/3639769002/jobs/6143576989#step:7:16)3 (info): Possible misspelling: CROSS_COMPILE may not be assigned. Did you mean CROSS_COMPILER?

For more information:
  https://www.shellcheck.net/wiki/SC2153 -- Possible misspelling: CROSS_COMPI...
task: Failed to run task "shell:check": exit status 1

[per1234]

Thanks Matteo!