Add pretty-error page infrastructure (#436)

This adds Miniflare 2's pretty-error page powered by
[Youch](https://github.com/poppinss/youch) to Miniflare 3.

Unfortunately, due to a bug in `workerd`, errors thrown
asynchronously by native APIs don't have `stack`s. This means we
can't extract the `stack` trace from dispatching to the user worker
by `try`/`catch`.

As a stop-gap solution, if the `MF-Experimental-Error-Stack` header
exists and is truthy on the response from the user worker, the body
will be interpreted as a JSON-error of the form
`{ message?: string, name?: string, stack?: string }`. `stack` will
be source-mapped if possible.

Another issue is that `workerd` gives all service-worker scripts the
name "worker.js", so if multiple service-workers are defined, we
can't identify which one threw. In this case, we don't display
sources in the pretty-error page.

Hopefully, we can fix both of these issues in `workerd`. We should be
able to reuse most of this infrastructure with `try`/`catch`s too.

packages/tre/README.md

@@ -223,6 +223,11 @@ parameter in module format Workers.
     must also be defined. Note the first module must be the entrypoint and have
     type `"ESModule"`.
 
+- `modulesRoot?: string`
+
+  If `modules` is set to an array, modules' "name"s will be their `path`s
+  relative to this value. This ensures file paths in stack traces are correct.
+
 <!-- prettier-ignore-start -->
 <!-- (for disabling `;` insertion in `js` code block) -->
 

packages/tre/package.json

@@ -35,10 +35,12 @@
     "glob-to-regexp": "^0.4.1",
     "http-cache-semantics": "^4.1.0",
     "kleur": "^4.1.5",
+    "source-map": "^0.7.4",
     "stoppable": "^1.1.0",
     "undici": "^5.12.0",
     "workerd": "^1.20221111.5",
     "ws": "^8.11.0",
+    "youch": "^3.2.2",
     "zod": "^3.18.0"
   },
   "devDependencies": {

packages/tre/src/index.ts

@@ -27,7 +27,12 @@ import {
   maybeGetSitesManifestModule,
   normaliseDurableObject,
 } from "./plugins";
-import { HEADER_CUSTOM_SERVICE, getUserServiceName } from "./plugins/core";
+import {
+  HEADER_CUSTOM_SERVICE,
+  SourceOptions,
+  getUserServiceName,
+  handlePrettyErrorRequest,
+} from "./plugins/core";
 import {
   Config,
   Runtime,
@@ -345,6 +350,11 @@ export class Miniflare {
         request,
         customService
       );
+    } else if (url.pathname === "/core/error") {
+      const workerSrcOpts = this.#workerOpts.map<SourceOptions>(
+        ({ core }) => core
+      );
+      response = await handlePrettyErrorRequest(workerSrcOpts, request);
     } else {
       // TODO: check for proxying/outbound fetch header first (with plans for fetch mocking)
       response = await this.#handleLoopbackPlugins(request, url);