Node version (or tell us if you're using electron or some other framework):

v16.1.0 and v14.17.0 (I tested both).

ShellJS version (the most recent version/Github branch you see the bug on):

I observed this while cloning both from my own fork and also cloning directly from the ShellJS master branch.

Operating system:

Ubuntu 16.04

Description of the bug:

Without making any code changes, I am getting an "import/extensions" linting error on npm run lint. The error says 'Unexpected use of file extension "js" for "../src/exec-child.js"', and references this test file/code line:

I see that the linting Travis CI tests passed recently (April 15), so I'm guessing there must have been some recent changes within a linting dependency (maybe eslint-plugin-import). Regardless, this looks like a simple fix, and I would be happy to file a PR.