fix: add EventSource typings (#21908)

Fixes #21691

ext/fetch/lib.deno_fetch.d.ts

@@ -405,3 +405,88 @@ declare function fetch(
   input: URL | Request | string,
   init?: RequestInit,
 ): Promise<Response>;
+
+/**
+ * @category Fetch API
+ */
+declare interface EventSourceInit {
+  withCredentials?: boolean;
+}
+
+/**
+ * @category Fetch API
+ */
+declare interface EventSourceEventMap {
+  "error": Event;
+  "message": MessageEvent;
+  "open": Event;
+}
+
+/**
+ * @category Fetch API
+ */
+declare interface EventSource extends EventTarget {
+  onerror: ((this: EventSource, ev: Event) => any) | null;
+  onmessage: ((this: EventSource, ev: MessageEvent) => any) | null;
+  onopen: ((this: EventSource, ev: Event) => any) | null;
+  /**
+   * Returns the state of this EventSource object's connection. It can have the values described below.
+   */
+  readonly readyState: number;
+  /**
+   * Returns the URL providing the event stream.
+   */
+  readonly url: string;
+  /**
+   * Returns true if the credentials mode for connection requests to the URL providing the event stream is set to "include", and false otherwise.
+   */
+  readonly withCredentials: boolean;
+  /**
+   * Aborts any instances of the fetch algorithm started for this EventSource object, and sets the readyState attribute to CLOSED.
+   */
+  close(): void;
+  readonly CONNECTING: 0;
+  readonly OPEN: 1;
+  readonly CLOSED: 2;
+  addEventListener<K extends keyof EventSourceEventMap>(
+    type: K,
+    listener: (this: EventSource, ev: EventSourceEventMap[K]) => any,
+    options?: boolean | AddEventListenerOptions,
+  ): void;
+  addEventListener(
+    type: string,
+    listener: (this: EventSource, event: MessageEvent) => any,
+    options?: boolean | AddEventListenerOptions,
+  ): void;
+  addEventListener(
+    type: string,
+    listener: EventListenerOrEventListenerObject,
+    options?: boolean | AddEventListenerOptions,
+  ): void;
+  removeEventListener<K extends keyof EventSourceEventMap>(
+    type: K,
+    listener: (this: EventSource, ev: EventSourceEventMap[K]) => any,
+    options?: boolean | EventListenerOptions,
+  ): void;
+  removeEventListener(
+    type: string,
+    listener: (this: EventSource, event: MessageEvent) => any,
+    options?: boolean | EventListenerOptions,
+  ): void;
+  removeEventListener(
+    type: string,
+    listener: EventListenerOrEventListenerObject,
+    options?: boolean | EventListenerOptions,
+  ): void;
+}
+
+/**
+ * @category Fetch API
+ */
+declare var EventSource: {
+  prototype: EventSource;
+  new (url: string | URL, eventSourceInitDict?: EventSourceInit): EventSource;
+  readonly CONNECTING: 0;
+  readonly OPEN: 1;
+  readonly CLOSED: 2;
+};