feat: add node:string_decoder with global polyfill

Author: pi0

README.md

@@ -77,7 +77,7 @@ Using this preset, we can convert a code that is depending on Node.js to work an
 | [node:stream/consumers](https://nodejs.org/api/stream.html)                 | Mocked     | [unenv/node/stream/consumers](./src/runtime/node/stream/consumers) |
 | [node:stream/promises](https://nodejs.org/api/stream.html)                  | Mocked     | [unenv/node/stream/promises](./src/runtime/node/stream/promises)   |
 | [node:stream/web](https://nodejs.org/api/stream.html)                       | Native     | [unenv/node/stream/web](./src/runtime/node/stream/web)             |
-| [node:string_decoder](https://nodejs.org/api/string_decoder.html)           | Mocked     | -                                                                  |
+| [node:string_decoder](https://nodejs.org/api/string_decoder.html)           | Polyfilled | [unenv/node/string_decoder](./src/runtime/node/string_decoder)     |
 | [node:sys](https://nodejs.org/api/sys.html)                                 | Mocked     | -                                                                  |
 | [node:timers](https://nodejs.org/api/timers.html)                           | Mocked     | -                                                                  |
 | [node:timers/promises](https://nodejs.org/api/timers.html)                  | Mocked     | -                                                                  |

src/presets/nodeless.ts

@@ -24,6 +24,7 @@ const nodeless: Preset & { alias: Map<string, string> } = {
         "stream/promises",
         "stream/consumers",
         "stream/web",
+        "string_decoder",
         "url",
         "util",
         "util/types",

src/runtime/node/string_decoder/index.ts

@@ -0,0 +1,11 @@
+// https://nodejs.org/api/string_decoder.html
+import type stringDecoder from "node:string_decoder";
+import { notImplementedClass } from "src/runtime/_internal/utils";
+
+export const StringDecoder: typeof stringDecoder.StringDecoder =
+  (globalThis as any).StringDecoder ||
+  notImplementedClass("string_decoder.StringDecoder");
+
+export default <typeof stringDecoder>{
+  StringDecoder,
+};