Reference .exists() in File-IO Docs (#9957)

* add mention of .exists()

* show that the exists method returns a promise in the docs

* remove unnecessary white space

* update type ref to show that exists returns a promise

docs/api/file-io.md

@@ -43,6 +43,7 @@ A `BunFile` can point to a location on disk where a file does not exist.
 const notreal = Bun.file("notreal.txt");
 notreal.size; // 0
 notreal.type; // "text/plain;charset=utf-8"
+const exists = await notreal.exists(); // false
 ```
 
 The default MIME type is `text/plain;charset=utf-8`, but it can be overridden by passing a second argument to `Bun.file`.
@@ -340,6 +341,7 @@ interface BunFile {
   arrayBuffer(): Promise<ArrayBuffer>;
   json(): Promise<any>;
   writer(params: { highWaterMark?: number }): FileSink;
+  exists(): Promise<boolean>;
 }
 
 export interface FileSink {