Make the type of the plugin ES module

[kentaroi]

Currently, the --experimental-require-module option is required to use eleventy-sass with Eleventy 3. While this option is no longer necessary in the current unstable Node.js(v23.x), the stable version (v24.x) is not expected to be released until the second quarter of 2025.

Therefore, I'd like to rewrite the entire plugin as an ES module instead of CommonJS. However, I am currently busy, so this might take some time.

cf. https://github.com/kentaroi/eleventy-sass/issues/30