shift asset processing to webpack (#424)

also migrated SASS to latest syntax (via dart-sass) and vastly simplified light/dark theme logic

gulpfile.js

@@ -0,0 +1,92 @@
+const gulp = require("gulp");
+const execa = require("gulp-execa").task;
+const htmlmin = require("gulp-html-minifier-terser");
+const imagemin = require("gulp-imagemin");
+const del = require("del");
+
+// use up-to-date imagemin plugins instead of those bundled with gulp-imagemin:
+const imageminMozjpeg = require("imagemin-mozjpeg");
+const imageminPngquant = require("imagemin-pngquant");
+const imageminGifsicle = require("imagemin-gifsicle");
+const imageminSvgo = require("imagemin-svgo");
+
+exports.default = gulp.series(
+  clean,
+  npx("webpack", ["--mode", "production", "--profile"]),
+  npx("hugo"),
+  gulp.parallel(
+    optimizeHtml,
+    optimizeImages,
+  ),
+);
+
+exports.serve = gulp.parallel(
+  npx("webpack", ["serve"]),
+  npx("hugo", ["--watch", "--buildDrafts", "--buildFuture", "--verbose"]),
+);
+
+exports.clean = gulp.task(clean);
+
+function optimizeHtml() {
+  return gulp
+    .src("public/**/*.html", { base: "./" })
+    .pipe(
+      htmlmin(
+        {
+          html5: true,
+          preserveLineBreaks: true,
+          collapseWhitespace: true,
+          collapseBooleanAttributes: true,
+          removeComments: true,
+          minifyCSS: false,
+          minifyJS: false,
+        }
+      )
+    )
+    .pipe(gulp.dest(".", { overwrite: true }));
+}
+
+function optimizeImages() {
+  return gulp
+    .src(["public/**/*.{gif,jpg,png,svg}", "!public/assets/emoji/*"], { base: "./" })
+    .pipe(
+      imagemin([
+        imageminMozjpeg({
+          quality: 85,
+          progressive: true,
+        }),
+        imageminPngquant({
+          quality: [0.7, 0.9],
+          speed: 1,
+          strip: true,
+        }),
+        imageminGifsicle(),
+        imageminSvgo(),
+      ],
+      {
+        verbose: true,
+      })
+    )
+    .pipe(gulp.dest(".", { overwrite: true }));
+}
+
+function clean() {
+  return del([
+    "public/",
+    "builds/",
+    "_vendor/",
+    "static/assets/",
+    "data/manifest.json",
+  ]);
+}
+
+// run a locally installed (i.e. ./node_modules/.bin/foo) binary, similar to a package.json script
+function npx(bin, options) {
+  // WARNING: MAJOR HACKS AHEAD:
+  const cmd = `${bin} ${options ? options.join(" ") : ""}`.trim();
+  return execa(cmd, {
+    preferLocal: true,
+    shell: true,
+    stdio: "inherit",
+  });
+}