doc: modernize and fix code examples in modules.md

* Replace `var` by `const`.
* Fix semicolons.
* Add missing code marks.
* Unify quotes.
* Comment out ellipsis.
* Use object destructuring.
* Use exponentiation operator.
* Replace snake_case by camelCase.

PR-URL: https://github.com/nodejs/node/pull/12224


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>