Extract the playground in its own HTML file

[vhiairrassary]

I have found src/include/play.h very complex to audit (it loads JS scripts from 3rd party sources) so I have extracted and formatted it in its own HTML file. Wdyt? I am happy to discard the PR if it does not make sense for you

[lmangani]

This was the initial noob hack to have the play UI embedded instead of it being served from a file to keep the application stand-alone. If this method retains the same approach (as it seems) and allows serving the UI without requiring it on the filesystem or sacrificing its readability, then it's nothing short of fantastic! TIL

[lmangani]

Since we're adding flexibility to this part, maybe we could also add an ENV option to allow loading any static UI from arbitrary local path as an alternative to the embedded play UI? Might be overwill but could be useful for customization.

[vhiairrassary]

If this method retains the same approach (as it seems)

Yes it should be the same (and maybe at some point in the future we could move to #embed or std::embed)

add an ENV option to allow loading any static UI from arbitrary local path

Sounds like a good idea, but I would prefer to focus on parameters before, wdyt? Do we merge this one as is?