Integrate with # shellcheck shell=... annotations

[mcandre]

Summary

When the shebang interpreter is not sufficient to select a syntax varient, then check for a # shellcheck shell=<interpeter> comment. These are commonly used to inform the ShellCheck linter system of which syntax to use, so it would make sense for the text editor to apply the same syntax.

Motivation

Many shell scripts like ash and pdksh are not currently highlighting as shell scripts, but they could easily be if we integrated with this ShellCheck annotation.

Describe alternatives you've considered

Manually configuring dozens of shell variants in Atom.