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-svg.js

@@ -0,0 +1,71 @@
+#!/usr/bin/env node
+import fastGlob from 'fast-glob';
+import {optimize} from 'svgo';
+import {parse} from 'node:path';
+import {readFile, writeFile, mkdir} from 'node:fs/promises';
+import {fileURLToPath} from 'node:url';
+import {exit} from 'node:process';
+
+const glob = (pattern) => fastGlob.sync(pattern, {
+  cwd: fileURLToPath(new URL('..', import.meta.url)),
+  absolute: true,
+});
+
+function doExit(err) {
+  if (err) console.error(err);
+  exit(err ? 1 : 0);
+}
+
+async function processFile(file, {prefix, fullName} = {}) {
+  let name;
+  if (fullName) {
+    name = fullName;
+  } else {
+    name = parse(file).name;
+    if (prefix) name = `${prefix}-${name}`;
+    if (prefix === 'octicon') name = name.replace(/-[0-9]+$/, ''); // chop of '-16' on octicons
+  }
+
+  // Set the `xmlns` attribute so that the files are displayable in standalone documents
+  // The svg backend module will strip the attribute during startup for inline display
+  const {data} = optimize(await readFile(file, 'utf8'), {
+    plugins: [
+      {name: 'preset-default'},
+      {name: 'removeDimensions'},
+      {name: 'prefixIds', params: {prefix: () => name}},
+      {name: 'addClassesToSVGElement', params: {classNames: ['svg', name]}},
+      {
+        name: 'addAttributesToSVGElement', params: {
+          attributes: [
+            {'xmlns': 'http://www.w3.org/2000/svg'},
+            {'width': '16'}, {'height': '16'}, {'aria-hidden': 'true'},
+          ]
+        }
+      },
+    ],
+  });
+
+  await writeFile(fileURLToPath(new URL(`../public/assets/img/svg/${name}.svg`, import.meta.url)), data);
+}
+
+function processFiles(pattern, opts) {
+  return glob(pattern).map((file) => processFile(file, opts));
+}
+
+async function main() {
+  try {
+    await mkdir(fileURLToPath(new URL('../public/assets/img/svg', import.meta.url)), {recursive: true});
+  } catch {}
+
+  await Promise.all([
+    ...processFiles('node_modules/@primer/octicons/build/svg/*-16.svg', {prefix: 'octicon'}),
+    ...processFiles('web_src/svg/*.svg'),
+    ...processFiles('public/assets/img/gitea.svg', {fullName: 'gitea-gitea'}),
+  ]);
+}
+
+try {
+  doExit(await main());
+} catch (err) {
+  doExit(err);
+}