chore: Goodbye gulp (#11447)

* Add a script for building assets

* Replace with script executions

* Remove gulp dependencies

* Fix dependencies

---------

Co-authored-by: Acid Chicken (硫酸鶏) <root@acid-chicken.com>
Co-authored-by: syuilo <Syuilotan@yahoo.co.jp>

scripts/dev.mjs

@@ -23,7 +23,13 @@ await execa('pnpm', ['build-pre'], {
 	stderr: process.stderr,
 });
 
-execa('pnpm', ['exec', 'gulp', 'watch'], {
+await execa('pnpm', ['build-assets'], {
+	cwd: _dirname + '/../',
+	stdout: process.stdout,
+	stderr: process.stderr,
+});
+
+execa('pnpm', ['build-assets', '--watch'], {
 	cwd: _dirname + '/../',
 	stdout: process.stdout,
 	stderr: process.stderr,