Follow up #33748
Now there are 3 "global" functions:
* registerGlobalSelectorFunc: for all elements matching the selector, eg: `.ui.dropdown`
* registerGlobalInitFunc: for `data-global-init="initInputAutoFocusEnd"`
* registerGlobalEventFunc: for `data-global-click="onCommentReactionButtonClick"`
And introduce `initGlobalInput` to replace old `initAutoFocusEnd` and
`attachDirAuto`, use `data-global-init` to replace fragile
`.js-autofocus-end` selector.
Another benefit is that by the new approach, no matter how many times
`registerGlobalInitFunc` is called, we only need to do one
"querySelectorAll" in the last step, it could slightly improve the
performance.
1. Rewrite `dirauto.ts` to `observer.ts`.
* We have been using MutationObserver for long time, it's proven that it
is quite performant.
* Now we extend its ability to handle more "init" works.
2. Use `observeAddedElement` to init all non-custom "dropdown".
3. Use `data-global-click` to handle click events from dynamically
loaded elements.
* By this new approach, the old fragile selector-based
(`.comment-reaction-button`) mechanism is removed.
4. By the way, remove unused `.diff-box` selector, it was abused and
never really used.
A lot of FIXMEs in "repo-diff.ts" are completely fixed, newly loaded
contents could work as expected.
