Add a standalone Modal layout

[MarcSkovMadsen]

I know the Templates have a modal. But its also powerful to be able to use and test it in notebooks. And its also much easier to search and learn about it as a separate component.

I've had such a great success with https://github.com/awesome-panel/panel-modal for my own Apps at work. Its just so easy to use and it always works.

There are some small changes in the api I would make and some more features from the underlying js library I would support if contributed.

I would like to contribute it to Panel if you would be willing to accept it?

[mattpap]

To note, bokeh 3.x has Dialog model, underdeveloped and currently used only for ExamineTool, but can be a basis for a useful component in the future.

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