Adds abort signal to requests

Enables request abortion using AbortSignal in all transports.
The change ensures requests can be cancelled,
prevents memory leaks and improves user experience by rejecting requests that are no longer relevant.

Implements AbortError class to differentiate RPC errors from aborts.
Ran `npm i baseline-browser-mapping@latest -D` to update browser mappings, and `npm audit fix` to fix 3 vulnerabilities (1 low, 1 moderate, 1 high).

Author: francislavoie

README.md

@@ -120,6 +120,40 @@ main().then(() => {
 
 </details>
 
+<details>
+  <summary>AbortController</summary>
+
+```javascript
+import { RequestManager, Client, HTTPTransport, AbortError } from "@open-rpc/client-js";
+
+const transport = new HTTPTransport("http://localhost:3333");
+const requestManager = new RequestManager([transport]);
+const client = new Client(requestManager);
+
+const main = async () => {
+  const controller = new AbortController();
+  const signal = controller.signal;
+
+  setTimeout(() => {
+    controller.abort();
+  }, 100);
+
+  try {
+    await client.request({ method: "long_running_method", params: [], timeout: 5000, signal });
+  } catch (e) {
+    if (e instanceof AbortError) {
+      console.log("Request was aborted");
+    } else {
+      console.error(e);
+    }
+  }
+};
+
+main();
+```
+
+</details>
+
 ### Building
 
 ```sh