Enable all webpack sourcemaps in dev build, disable all in prod build (#25089)

- Enable all source maps in dev build
- Disable all source maps in prod build
- Provide `ENABLE_SOURCEMAP` env var to override it.

I think the strange error seen in
https://github.com/go-gitea/gitea/issues/24784 is sourcemap related, so
if we enable/disable them all, it might go away. But it's most
definitely a Safari bug.

With all sourcemaps disabled, binary size goes down by around 1-2 MB,
with all enabled it goes up by around 12MB. If +12MB is acceptable, we
could also always enable them by default as fully source maps do have
some debugging benefits.

docs/content/doc/installation/from-source.en-us.md

 TAGS="bindata" make backend
 ```
 
+Webpack source maps are by default enabled in development builds and disabled in production builds. They can be enabled by setting the `ENABLE_SOURCEMAP=true` environment variable.
+
 ## Test
 
 After following the steps above, a `gitea` binary will be available in the working directory.

webpack.config.js

 import webpack from 'webpack';
 import {fileURLToPath} from 'node:url';
 import {readFileSync} from 'node:fs';
+import {env} from 'node:process';
 
 const {EsbuildPlugin} = EsBuildLoader;
 const {SourceMapDevToolPlugin, DefinePlugin} = webpack;
   themes[parse(path).name] = [path];
 }
 
-const isProduction = process.env.NODE_ENV !== 'development';
+const isProduction = env.NODE_ENV !== 'development';
+
+let sourceMapEnabled;
+if ('ENABLE_SOURCEMAP' in env) {
+  sourceMapEnabled = env.ENABLE_SOURCEMAP === 'true';
+} else {
+  sourceMapEnabled = !isProduction;
+}
 
 const filterCssImport = (url, ...args) => {
   const cssFile = args[1] || args[0]; // resourcePath is 2nd argument for url and 3rd for import
           {
             loader: 'css-loader',
             options: {
-              sourceMap: true,
+              sourceMap: sourceMapEnabled,
               url: {filter: filterCssImport},
               import: {filter: filterCssImport},
             },
       filename: 'css/[name].css',
       chunkFilename: 'css/[name].[contenthash:8].css',
     }),
-    new SourceMapDevToolPlugin({
+    sourceMapEnabled && (new SourceMapDevToolPlugin({
       filename: '[file].[contenthash:8].map',
-      include: [
-        'js/index.js',
-        'css/index.css',
-      ],
-    }),
+    })),
     new MonacoWebpackPlugin({
       filename: 'js/monaco-[name].[contenthash:8].worker.js',
     }),
       emitError: true,
       allow: '(Apache-2.0 OR BSD-2-Clause OR BSD-3-Clause OR MIT OR ISC OR CPAL-1.0 OR Unlicense OR EPL-1.0 OR EPL-2.0)',
     }) : new AddAssetPlugin('js/licenses.txt', `Licenses are disabled during development`),
-  ],
+  ].filter(Boolean),
   performance: {
     hints: false,
     maxEntrypointSize: Infinity,