Fixes #366: Add compatibility for using CommonJs

[jheagle]

Fixes #366

Add compatibility for using CommonJs since this package is intended for GulpJs which does yet support ES6,

• Install common-exports to be able to convert the package to CommonJs.
• Update the package to not be type=module.
• Update the file extensions to .mjs (I also don't like this, but the alternative of .cjs was not compatible)
• Add a gulpfile.js and babel.config.js to be able to run common-exports.
• Create the CommonJs files in a new cjs directory.
• Add a duplicate of the test.js file to be able to test the CommonJs compiled files.
• Add signal-exit since the latest version is a dependency of mozjpeg to prevent failures with onExit missing.

[sindresorhus]

Thanks, but Gulp does support ESM.