add support for svgo.config.cjs files in esm environments#879

<!-- Thanks for submitting a pull request! Please provide enough information so that others can review your pull request. The two fields below are mandatory. --> ## Summary <!-- Explain the **motivation** for making this change. What existing problem does the pull request solve? --> In our vite project, since it is `"type": "module"` (ESM), the cli wouldn't accept svgo config files as a regular `.js` file that wasn't esm. This PR enables the cli to use `.cjs`. Addresses ## Test plan <!-- Demonstrate the code is solid. Example: The exact commands you ran and their output, screenshots / videos if the pull request changes UI. --> - I ran the modified code in our vite environment and it accepted `svgo.config.cjs` - I've added a test. - I wasn't able to add error checking for ESM detection as I think jest is doing some funny things there? Thank you for the wonderful library and let me know if I can do anything else to this PR 🎉