feat: Expose nodes that are being highlighted

[xxholly32]

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What problem does this feature solve?

In readonly mode, the user can select the corresponding node, which often needs to obtain the information of the user's click in another scenario. So I wonder if I can throw an event to the outside when the user clicks on the node

What does the proposed API look like?

Name	Description	Type
click	triggers after a node click	^[Function]`(node: any, clickNode:string === 'key'

[cloydlau]

Hello, I think you should be able to find some relevant APIs in the doc.
See josdejong/svelte-jsoneditor#163
Or

document.querySelector('.jse-contents').addEventListener('click', (e) => {
  console.log(e.target.innerText)
})