export unset_variable should trigger SC2154

[kbulygin]

For bugs

• Rule Id (if any, e.g. SC1000):
• My shellcheck version (shellcheck --version or 'online'): online
• I tried on shellcheck.net and verified that this is still a problem on the latest commit

For new checks and feature suggestions

• shellcheck.net (i.e. the latest commit) currently gives no useful warnings about this
• I searched through https://github.com/koalaman/shellcheck/issues and didn't find anything related

Here's a snippet or screenshot that shows the problem:

#!/bin/bash
export x

Here's what shellcheck currently says:

Nothing

Here's what I wanted or expected to see:

SC2154: x is referenced but not assigned.