Remove jQuery from repo wiki creation page (#29271)

- Switched to plain JavaScript
- Tested the wiki creation form functionality and it works as before

# Demo using JavaScript without jQuery

![action](https://github.com/go-gitea/gitea/assets/20454870/2dfc95fd-40cc-4ffb-9ae6-50f798fddd67)

---------

Signed-off-by: Yarden Shoham <git@yardenshoham.com>
Co-authored-by: silverwind <me@silverwind.io>

web_src/js/utils/dom.js

@@ -227,3 +227,15 @@ export function initSubmitEventPolyfill() {
   document.body.addEventListener('click', submitEventPolyfillListener);
   document.body.addEventListener('focus', submitEventPolyfillListener);
 }
+
+/**
+ * Check if an element is visible, equivalent to jQuery's `:visible` pseudo.
+ * Note: This function doesn't account for all possible visibility scenarios.
+ * @param {HTMLElement} element The element to check.
+ * @returns {boolean} True if the element is visible.
+ */
+export function isElemVisible(element) {
+  if (!element) return false;
+
+  return Boolean(element.offsetWidth || element.offsetHeight || element.getClientRects().length);
+}