Allow opening popup contents in a temporary buffer

[epbuennig]

I have recently used :tree-sitter-subtree and it showed a fairly large subtree in the popup.
I'm not the author of the tree sitter grammar and wanted to include the tree in an issue.
But there's currently no way to actually copy the content of such a popup or to write it to a file or directly into a buffer.

Ideally that should be possible for any popup, it would allow copying examples from a documentation popup, or very large outputs for such debugging commands.

The tree in question:

Unfortunately, #4295 was not merged before 22.08.1, which is why I wanted to copy the tree, to format it.

I personally suggest allowing opening a temporary buffer for a popup to allow granular selection of what to copy, I don't have any strong feelings about possible key binds.

I wasn't sure if this should be a C-enhancement issue, so I opened a discussion instead, as this could be seen as a feature request.
If this falls under C-enhacements I'll open a proper issue for it.