Remove the through2 dependency in favor of the built-in Node.js stream.Transform

The `through2` dependency got introduced over four years ago in #11325 to
replace the unmaintained `gulp-transform` dependency. However, sadly the
same holds for `through2` since the last release was also four years ago.

Fortunately the `through2` dependency can trivially be replaced with the
built-in Node.js `stream.Transform` API nowadays. In fact, the `through2`
dependency mentions themselves in their README already that they are "a
tiny wrapper around Node.js streams.Transform". The `stream.Transform`
API is available in all Node.js versions we support, and in Node.js 6
already the simplified constructor approach for `stream.Transform` got
introduced to simplify creating custom stream transformers; see
https://nodejs.org/docs/latest-v6.x/api/stream.html#stream_new_stream_transform_options.

This commit therefore replaces `through2` by switching to the
`stream.Transform` API directly so we don't need any wrappers anymore.
Note that for our case the only change we have to make is to enable
object mode, see https://nodejs.org/api/stream.html#object-mode, because
we pass in `VinylFile` objects instead of e.g. regular `Buffer` objects.

I have confirmed in two ways that this is indeed a drop-in replacement:

- Running the Gulp targets that call the `transform` function and
  diffing the resulting `build` folder before/after this patch, with
  `diff -r build-old/ build-new/`, to ensure that there are no
  unexpected changes in the output.
- Changing the Gulpfile to, instead of UTF-8, transform the files to
  ASCII, and diffing the resulting `build` folder to confirm that the
  transformation logic works and produces different results, such as:

```
diff build/lib/core/standard_fonts.js build-ascii/lib/core/standard_fonts.js
284c284
<   t["Trinité"] = true;
---
>   t["Trinit�"] = true;
```

gulpfile.mjs

@@ -38,7 +38,6 @@ import replace from "gulp-replace";
 import stream from "stream";
 import streamqueue from "streamqueue";
 import TerserPlugin from "terser-webpack-plugin";
-import through from "through2";
 import Vinyl from "vinyl";
 import webpack2 from "webpack";
 import webpackStream from "webpack-stream";
@@ -119,13 +118,16 @@ const DEFINES = Object.freeze({
 });
 
 function transform(charEncoding, transformFunction) {
-  return through.obj(function (vinylFile, enc, done) {
-    const transformedFile = vinylFile.clone();
-    transformedFile.contents = Buffer.from(
-      transformFunction(transformedFile.contents),
-      charEncoding
-    );
-    done(null, transformedFile);
+  return new stream.Transform({
+    objectMode: true,
+    transform(vinylFile, enc, done) {
+      const transformedFile = vinylFile.clone();
+      transformedFile.contents = Buffer.from(
+        transformFunction(transformedFile.contents),
+        charEncoding
+      );
+      done(null, transformedFile);
+    },
   });
 }