Add jest config and polyfills

jest.config.js

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+module.exports = {
+    setupFiles: ['./jest.polyfills.js'],
+}

jest.polyfills.js

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+/**
+ * @note The block below contains polyfills for Node.js globals
+ * required for Jest to function when running JSDOM tests.
+ * These HAVE to be require's and HAVE to be in this exact
+ * order, since "undici" depends on the "TextEncoder" global API.
+ *
+ * Consider migrating to a more modern test runner if
+ * you don't want to deal with this.
+ */
+
+const { TextDecoder, TextEncoder, ReadableStream } = require("node:util")
+
+Object.defineProperties(globalThis, {
+    TextDecoder: { value: TextDecoder },
+    TextEncoder: { value: TextEncoder },
+    ReadableStream: { value: ReadableStream },
+})
+
+const { Blob, File } = require("node:buffer")
+const { fetch, Headers, FormData, Request, Response } = require("undici")
+
+Object.defineProperties(globalThis, {
+    fetch: { value: fetch, writable: true },
+    Blob: { value: Blob },
+    File: { value: File },
+    Headers: { value: Headers },
+    FormData: { value: FormData },
+    Request: { value: Request },
+    Response: { value: Response },
+});