Replace request library with undici

The [request](https://www.npmjs.com/package/request) library has been deprecated for some time. This change:

1. Replaces use of request (the library) with the `request` function from [undici](https://github.com/nodejs/undici), which has been adopted by Node to provide an implementation of the now-standard [`fetch`](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API) web API. This `request` API is almost but not quite identical to `fetch`, but undici's `fetch` is not available for some of our currently supported Node versions.

2. Refactors the three very similar dev-facing API endpoints provided by enketo-transformer's `app.js`. This isn't strictly necessary but it was easier to reason about than applying the same changes repeatedly, and...

3. ... caught/fixed a bug where the POST handler always returned a 400 Bad Request for valid POST bodies with an `xform` property.

.eslintrc.json

@@ -11,7 +11,7 @@
     "plugins": ["chai-friendly", "jsdoc", "prettier", "unicorn"],
     "parserOptions": {
         "sourceType": "module",
-        "ecmaVersion": 2019
+        "ecmaVersion": 2020
     },
     "settings": {
         "jsdoc": {