Move some JS code from fomantic.js to standalone files (#27994)

To improve maintainability, this PR: 

1. Rename `web_src/js/modules/aria` to `web_src/js/modules/fomantic`
(the code there are all for aria of fomantic)
2. Move api/transition related code to
`web_src/js/modules/fomantic/api.js` and
`web_src/js/modules/fomantic/transition.js`

No logic is changed.

web_src/js/modules/fomantic/checkbox.js

@@ -0,0 +1,38 @@
+import $ from 'jquery';
+import {generateAriaId} from './base.js';
+
+const ariaPatchKey = '_giteaAriaPatchCheckbox';
+const fomanticCheckboxFn = $.fn.checkbox;
+
+// use our own `$.fn.checkbox` to patch Fomantic's checkbox module
+export function initAriaCheckboxPatch() {
+  if ($.fn.checkbox === ariaCheckboxFn) throw new Error('initAriaCheckboxPatch could only be called once');
+  $.fn.checkbox = ariaCheckboxFn;
+  ariaCheckboxFn.settings = fomanticCheckboxFn.settings;
+}
+
+// the patched `$.fn.checkbox` checkbox function
+// * it does the one-time attaching on the first call
+function ariaCheckboxFn(...args) {
+  const ret = fomanticCheckboxFn.apply(this, args);
+  for (const el of this) {
+    if (el[ariaPatchKey]) continue;
+    attachInit(el);
+  }
+  return ret;
+}
+
+function attachInit(el) {
+  // Fomantic UI checkbox needs to be something like: <div class="ui checkbox"><label /><input /></div>
+  // It doesn't work well with <label><input />...</label>
+  // To make it work with aria, the "id"/"for" attributes are necessary, so add them automatically if missing.
+  // In the future, refactor to use native checkbox directly, then this patch could be removed.
+  el[ariaPatchKey] = {}; // record that this element has been patched
+  const label = el.querySelector('label');
+  const input = el.querySelector('input');
+  if (!label || !input || input.getAttribute('id')) return;
+
+  const id = generateAriaId();
+  input.setAttribute('id', id);
+  label.setAttribute('for', id);
+}