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

.vscode/launch.json

@@ -0,0 +1,16 @@
+{
+  // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
+  "version": "0.2.0",
+  "configurations": [
+    {
+      "type": "node",
+      "request": "launch",
+      "name": "Debug Current Test File",
+      "autoAttachChildProcesses": true,
+      "program": "${workspaceRoot}/node_modules/vitest/vitest.mjs",
+      "args": ["run", "${relativeFile}"],
+      "smartStep": true,
+      "console": "integratedTerminal"
+    }
+  ]
+}