[Enhancement]: Reagent 2 / React 19 compatibility

[Deraen]

What do you suggest?

While 10x includes inlined copy of Reagent, that won't help if users update to React 19, as the old React-DOM APIs won't exist.

I guess it would be nice to keep also supporting old React for 10x. This might work by including Reagent 2, and then choosing to use either reagent.dom or reagent.dom.client by checking either React version or by checking if react-dom/render value exists.

I might take a stab at this.

[Deraen]

I found a note about React 18:

For React 18: Consider using {:preloads [day8.re-frame-10x.preload.react-18]}. This will use React's new render API. Otherwise, expect deprecation warnings from React.

This same solution will also work for Reagent 2 / React 19. Maybe this should just be changed into the default, and 18/17 compatibility mode would be behind a namespace.

Benefit of Reagent 2 vs 1.2 with React 18/19 concurrent mode, is that ratom changes are batched together from Reagent queue into React with flushSync.

[kimo-k]

I can foresee preload.react-18 working. As for making it our default: possibly. One outstanding problem with that react-18 preload is that the pop-out window still uses the old render api, and might be unstable. Next steps would be to enumerate the DX implications of the changing that default, and debug that pop-out window.