Add TypeScript types for TextEncoder and TextDecoder globals

Both TextEncoder (Hermes 0.74) and TextDecoder (Hermes 0.85) are
available at runtime but missing from the TypeScript globals, forcing
users to cast through any, pull in lib.dom, or write their own
declarations. Adds declarations matching the Hermes implementation and
expands the globals typetests to cover the new APIs.

Fixes #56325

Author: wildan-m

packages/react-native/src/types/globals.d.ts

@@ -521,6 +521,59 @@ declare global {
     ): URLSearchParams;
   };
 
+  interface TextEncoderEncodeIntoResult {
+    read: number;
+    written: number;
+  }
+
+  /**
+   * Encodes strings to bytes using UTF-8.
+   * Available in Hermes since React Native 0.74.
+   *
+   * [MDN Reference](https://developer.mozilla.org/docs/Web/API/TextEncoder)
+   */
+  interface TextEncoder {
+    /** Always `"utf-8"`. */
+    readonly encoding: string;
+    encode(input?: string): Uint8Array;
+    encodeInto(
+      source: string,
+      destination: Uint8Array,
+    ): TextEncoderEncodeIntoResult;
+  }
+
+  var TextEncoder: {
+    prototype: TextEncoder;
+    new (): TextEncoder;
+  };
+
+  interface TextDecoderOptions {
+    fatal?: boolean;
+    ignoreBOM?: boolean;
+  }
+
+  interface TextDecodeOptions {
+    stream?: boolean;
+  }
+
+  /**
+   * Decodes bytes to strings. Supports UTF-8 and several legacy encodings.
+   * Available in Hermes since React Native 0.85.
+   *
+   * [MDN Reference](https://developer.mozilla.org/docs/Web/API/TextDecoder)
+   */
+  interface TextDecoder {
+    readonly encoding: string;
+    readonly fatal: boolean;
+    readonly ignoreBOM: boolean;
+    decode(input?: BufferSource, options?: TextDecodeOptions): string;
+  }
+
+  var TextDecoder: {
+    prototype: TextDecoder;
+    new (label?: string, options?: TextDecoderOptions): TextDecoder;
+  };
+
   interface WebSocketMessageEvent extends Event {
     data?: any | undefined;
   }

packages/react-native/types/__typetests__/globals.tsx

@@ -216,3 +216,22 @@ const formData = new FormData();
 formData.append('file', { fileName: 'example' });
 console.log(formData.getParts());
 console.log(formData.getAll('username'));
+
+const textEncoder = new TextEncoder();
+const encodingValue: string = textEncoder.encoding;
+const encoded: Uint8Array = textEncoder.encode();
+const encoded2: Uint8Array = textEncoder.encode('hello');
+const encodeIntoResult = textEncoder.encodeInto('hello', new Uint8Array(10));
+const readCount: number = encodeIntoResult.read;
+const writtenCount: number = encodeIntoResult.written;
+
+const textDecoder = new TextDecoder();
+const textDecoderWithLabel = new TextDecoder('utf-8');
+const textDecoderWithOptions = new TextDecoder('utf-8', { fatal: true, ignoreBOM: false });
+const decoderEncoding: string = textDecoder.encoding;
+const decoderFatal: boolean = textDecoder.fatal;
+const decoderIgnoreBOM: boolean = textDecoder.ignoreBOM;
+const decoded: string = textDecoder.decode();
+const decoded2: string = textDecoder.decode(new Uint8Array([72, 101, 108, 108, 111]));
+const decoded3: string = textDecoder.decode(new Uint8Array([72, 101, 108, 108, 111]), { stream: true });
+const decoded4: string = textDecoder.decode(new ArrayBuffer(5));