Move custom `tw-` helpers to tailwind plugin (#31184)

Move the previous custom `tw-` classes to be defined in a tailwind
plugin. I think it's cleaner that way and I also verified double-class
works as expected:

<img width="299" alt="Screenshot 2024-05-30 at 19 06 24"
src="https://github.com/go-gitea/gitea/assets/115237/003cbc76-2013-46a0-9e27-63023fa7c7a4">

tailwind.config.js

@@ -1,6 +1,7 @@
 import {readFileSync} from 'node:fs';
 import {env} from 'node:process';
 import {parse} from 'postcss';
+import plugin from 'tailwindcss/plugin.js';
 
 const isProduction = env.NODE_ENV !== 'development';
 
@@ -98,4 +99,26 @@ export default {
       })),
     },
   },
+  plugins: [
+    plugin(({addUtilities}) => {
+      addUtilities({
+        // tw-hidden must win all other "display: xxx !important" classes to get the chance to "hide" an element.
+        // do not use:
+        // * "[hidden]" attribute: it's too weak, can not be applied to an element with "display: flex"
+        // * ".hidden" class: it has been polluted by Fomantic UI in many cases
+        // * inline style="display: none": it's difficult to tweak
+        // * jQuery's show/hide/toggle: it can not show/hide elements with "display: xxx !important"
+        // only use:
+        // * this ".tw-hidden" class
+        // * showElem/hideElem/toggleElem functions in "utils/dom.js"
+        '.hidden.hidden': {
+          'display': 'none',
+        },
+        // proposed class from https://github.com/tailwindlabs/tailwindcss/pull/12128
+        '.break-anywhere': {
+          'overflow-wrap': 'anywhere',
+        },
+      });
+    }),
+  ],
 };

web_src/css/helpers.css

@@ -35,22 +35,6 @@ Gitea's private styles use `g-` prefix.
 .interact-bg:hover { background: var(--color-hover) !important; }
 .interact-bg:active { background: var(--color-active) !important; }
 
-/*
-tw-hidden must win all other "display: xxx !important" classes to get the chance to "hide" an element.
-do not use:
-* "[hidden]" attribute: it's too weak, can not be applied to an element with "display: flex"
-* ".hidden" class: it has been polluted by Fomantic UI in many cases
-* inline style="display: none": it's difficult to tweak
-* jQuery's show/hide/toggle: it can not show/hide elements with "display: xxx !important"
-only use:
-* this ".tw-hidden" class
-* showElem/hideElem/toggleElem functions in "utils/dom.js"
-*/
-.tw-hidden.tw-hidden { display: none !important; }
-
-/* proposed class from https://github.com/tailwindlabs/tailwindcss/pull/12128 */
-.tw-break-anywhere { overflow-wrap: anywhere !important; }
-
 @media (max-width: 767.98px) {
   /* double selector so it wins over .tw-flex (old .gt-df) etc */
   .not-mobile.not-mobile {