feat: new command to open hover documentation in a new buffer instead of a popup

[NikitaRevenco]

So, you can use Space + k to hover documentation which opens a popup.

However, sometimes it's really useful to be able to search inside this popup. Or go into it, move around, copy some text, etc. Some hover-docs can be quite long.

This PR adds a new command called hover_dump, which dumps the "documentation on hover" into a new scratch buffer, where you can use your usual Helix motions. this is how it looks like (window on the right):

The red box is where my cursor was, and it opened the documentation in a new buffer instead of a popup.

I bound hover_dump to Space + K, whereas hover was Space + k.

Closes #12206
Closes #8134
Closes #9455