Add shells/modernish

The programming/scripting language that incorporates the most frustrating
combination of deficiencies and awesome power is probably the POSIX shell with
accompanying utilities, which all exist in several variant implementations. Due
to said power, the shell refuses to die as a scripting language. But when
scripters are bitten by arcane grammar pitfalls, defective tutorials on the
web, or shell functionality deficits, then 'use a real programming language
instead' is generally the mantra.

Enter modernish, a new vision on shell scripting. Modernish aims to provide a
standard library that allows for writing robust, portable, readable, and
powerful programs for POSIX-based shells and utilities. It can solve the shell
language problems mentioned above, and many more. With modernish, you'd almost
think the shell has become a modern programming language!

The library builds on the POSIX 2018 Edition standard, so it should run on any
sufficiently POSIX-compliant shell and operating system. But it also takes
advantage of certain shell-specific enhancements if it detects them.

WWW: https://github.com/modernish/modernish

Differential Revision: https://reviews.freebsd.org/D19350