Ignore some shell scripts, add files without shebang line

[bewuethr]

Currently, the check is for file output matching shell script. This is both including things it shouldn't (fish shell, C shell scripts) and excluding files without a shebang line.

The check for file output should be more precise (see file source code), and files identified by their extension should be included.

From the ShellCheck man page:

Specify Bourne shell dialect. Valid values are sh, bash, dash and ksh. The
default is to deduce the shell from the file's shell directive, shebang, or
.bash/.bats/.dash/.ksh extension, in that order. sh refers to POSIX sh (not
the system's), and will warn of portability issues.

[bewuethr]

Want:

• file output matching
  • POSIX shell script
  • Korn shell script
  • Bourne-Again shell script
  • /usr/bin/env sh script
  • /usr/bin/env ksh script
• filenames ending in
  • .sh
  • .bash
  • .ksh