Add macros sh and sh< for running commands in a real shell

[MaxGyver83]

Use these macros to run commands in a shell (sh). This allows using shell variables, cd and other shell builtins.

Examples:

(import sh)

(sh/sh cd /tmp && git clone https://github.com/andrewchambers/janet-sh)

(sh/sh<
  if [ "${USER}" = ,(os/getenv "USER") ];
  then echo "it's \"me\""; else echo "not me"; fi)

These things don't work:

• (sh/sh< echo 'test') (text in single quotes) → Use double quotes or no quotes at all.
• (sh/sh< echo hello\ world) (escaping spaces) → Write (sh/sh< echo "hello world") instead.
• (sh/sh< echo \$PWD) (escaping dollar signs) → Use (sh/sh< echo `\$PWD`) instead.
• Probably other things.

To be discussed

• The new function collect-proc-specs-subshell is very similar (and based on) collect-proc-specs. Potential code duplication!?
• Might be needed for sake of completeness: sh?, sh<_, bash, bash<, bash?, bash<_!?

This addresses #21 and #22.