Introduce CJS exports

ES modules cannot be imported (required) to a CJS modules. It's a pity
to be unable to use a module like this in a CJS project, especially
because offering a CJS build output next to the ES one is very easy.

The final solution would probably somehow make use of `@ehmicky/dev-tasks`,
but it'd mean more work. Consumers of this package won't notice any
difference, if it's done later.

Author: prantlf

.gitignore

@@ -10,3 +10,4 @@ node_modules
 !.github/
 /coverage
 /build
+/cjs