Migrate to ESM (#484)

* Migrate to ESM

* just use `require.resolve`

* reuse setup action

* checkout PR head for a sec

* ls things

* try a thing

* the dirname had a trailing whitespace, lol

.github/actions/ci-setup/action.yml

@@ -0,0 +1,14 @@
+name: Setup CI
+
+runs:
+  using: composite
+  steps:
+    - name: Setup Node.js
+      uses: actions/setup-node@v4
+      with:
+        node-version-file: ".node-version"
+        cache: yarn
+
+    - name: Install dependencies
+      shell: bash
+      run: yarn install --frozen-lockfile