Added $? equivalent to $env.LAST_EXIT_CODE

[RolandMarchand]

This pull request adds a nushell equivalent to a bash command in the Coming from Bash book chapter.

This is the added equivalence:

Bash	Nu	Task
echo $?	$env.LAST_EXIT_CODE	See the exit status of the last executed command

This equivalence was added because $? is very common in bash scripts.

The change can be previewed here: https://rolandmarchand.github.io/nushell.github.io/book/coming_from_bash.html#command-equivalents.

[fdncred]

Good catch. Thanks!