move some scripts from 'build' to 'tools' directory, misc refactors (#29844)

- Move some scripts from `build` to new `tools` dir. Eventually i would
like to move all but let's do it step-by-step.
- Add dir to eslint and move the files into vars.
- Update docs accordingly.
- While updating docs I noticed we were incorrectly having `public/img`
path still in a few places. Replace those with the current
`public/assets/img`.

---------

Co-authored-by: Nanguan Lin <nanguanlin6@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

tools/generate-images.js

@@ -0,0 +1,79 @@
+#!/usr/bin/env node
+import imageminZopfli from 'imagemin-zopfli'; // eslint-disable-line i/no-unresolved
+import {loadSVGFromString, Canvas, Rect, util} from 'fabric/node'; // eslint-disable-line i/no-unresolved
+import {optimize} from 'svgo';
+import {readFile, writeFile} from 'node:fs/promises';
+import {argv, exit} from 'node:process';
+
+function doExit(err) {
+  if (err) console.error(err);
+  exit(err ? 1 : 0);
+}
+
+async function generate(svg, path, {size, bg}) {
+  const outputFile = new URL(path, import.meta.url);
+
+  if (String(outputFile).endsWith('.svg')) {
+    const {data} = optimize(svg, {
+      plugins: [
+        'preset-default',
+        'removeDimensions',
+        {
+          name: 'addAttributesToSVGElement',
+          params: {attributes: [{width: size}, {height: size}]}
+        },
+      ],
+    });
+    await writeFile(outputFile, data);
+    return;
+  }
+
+  const {objects, options} = await loadSVGFromString(svg);
+  const canvas = new Canvas();
+  canvas.setDimensions({width: size, height: size});
+  const ctx = canvas.getContext('2d');
+  ctx.scale(options.width ? (size / options.width) : 1, options.height ? (size / options.height) : 1);
+
+  if (bg) {
+    canvas.add(new Rect({
+      left: 0,
+      top: 0,
+      height: size * (1 / (size / options.height)),
+      width: size * (1 / (size / options.width)),
+      fill: 'white',
+    }));
+  }
+
+  canvas.add(util.groupSVGElements(objects, options));
+  canvas.renderAll();
+
+  let png = Buffer.from([]);
+  for await (const chunk of canvas.createPNGStream()) {
+    png = Buffer.concat([png, chunk]);
+  }
+
+  png = await imageminZopfli({more: true})(png);
+  await writeFile(outputFile, png);
+}
+
+async function main() {
+  const gitea = argv.slice(2).includes('gitea');
+  const logoSvg = await readFile(new URL('../assets/logo.svg', import.meta.url), 'utf8');
+  const faviconSvg = await readFile(new URL('../assets/favicon.svg', import.meta.url), 'utf8');
+
+  await Promise.all([
+    generate(logoSvg, '../public/assets/img/logo.svg', {size: 32}),
+    generate(logoSvg, '../public/assets/img/logo.png', {size: 512}),
+    generate(faviconSvg, '../public/assets/img/favicon.svg', {size: 32}),
+    generate(faviconSvg, '../public/assets/img/favicon.png', {size: 180}),
+    generate(logoSvg, '../public/assets/img/avatar_default.png', {size: 200}),
+    generate(logoSvg, '../public/assets/img/apple-touch-icon.png', {size: 180, bg: true}),
+    gitea && generate(logoSvg, '../public/assets/img/gitea.svg', {size: 32}),
+  ]);
+}
+
+try {
+  doExit(await main());
+} catch (err) {
+  doExit(err);
+}