Enable TypeScript strictNullChecks (#35843)

A big step towards enabling strict mode in Typescript.

There was definitely a good share of potential bugs while refactoring
this. When in doubt, I opted to keep the potentially broken behaviour.
Notably, the `DOMEvent` type is gone, it was broken and we're better of
with type assertions on `e.target`.

---------

Signed-off-by: silverwind <me@silverwind.io>
Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/markup/codecopy.ts

@@ -17,6 +17,6 @@ export function initMarkupCodeCopy(elMarkup: HTMLElement): void {
     btn.setAttribute('data-clipboard-text', el.textContent.replace(/\r?\n$/, ''));
     // we only want to use `.code-block-container` if it exists, no matter `.code-block` exists or not.
     const btnContainer = el.closest('.code-block-container') ?? el.closest('.code-block');
-    btnContainer.append(btn);
+    btnContainer!.append(btn);
   });
 }