fix[devtools/tree/element]: onClick -> onMouseDown to handle first click correctly (#28486)

There is a weird behaviour in all shells of RDT: when user opens
`Components` tab and scrolls down a tree (without any prior click or
focus event), and then clicks on some element, the `click` event will
not be fired. Because `click` event hasn't been fired, the `focus` event
is fired for the whole list and we pre-select the first (root) element
in the tree:

https://github.com/facebook/react/blob/034130c02ffb47b0026059b57d17e9b080976ff3/packages/react-devtools-shared/src/devtools/views/Components/Tree.js#L217-L226

Check the demo (before) what is happening. I don't know exactly why
`click` event is not fired there, but it only happens:
1. For elements, which were not previously rendered (for virtualization
purposes).
2. When HTML-element (div), which represents the container for the tree
was not focused previously.

Unlike the `click` event, the `mousedown` event is fired consistently.

### Before


https://github.com/facebook/react/assets/28902667/9f3ad75d-55d0-4c99-b2d0-ead63a120ea0

### After



https://github.com/facebook/react/assets/28902667/e34816be-644c-444c-8e32-562a79494e44


Tested that it works in all shells, including the select / deselect
features (with `metaKey` param in event).