If you go to (eg) https://github.com/exercism/codemirror-lang-elixir?tab=readme-ov-file you’ll find:

import { StreamLanguage } from '@codemirror/language'
import { elixir } from 'codemirror-lang-elixir'

const lang = StreamLanguage.define(elixir) 

Or on https://github.com/livebook-dev/codemirror-lang-elixir

import { EditorView } from "@codemirror/view";
import { basicSetup } from "codemirror";
import { elixir } from "codemirror-lang-elixir";

new EditorView({
  parent: document.querySelector("#editor"),
  extensions: [basicSetup, elixir()],
});

What is that? Seriously, imagine I’m a five year old: what would you call it, what does it mean, where does it go, how could I ever actually run and test and in fact just see it in action?