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/components/ViewFileTreeItem.vue

@@ -4,7 +4,7 @@ import {isPlainClick} from '../utils/dom.ts';
 import {shallowRef} from 'vue';
 import {type createViewFileTreeStore} from './ViewFileTreeStore.ts';
 
-type Item = {
+export type Item = {
   entryName: string;
   entryMode: 'blob' | 'exec' | 'tree' | 'commit' | 'symlink' | 'unknown';
   entryIcon: string;