patch 9.1.1856: cannot style popup window (border, shadow, etc)

patch 9.1.1856: cannot style popup window (border, shadow, etc)

Problem:  cannot style popup window (border, shadow, etc)
Solution: Extend the 'completepopup' option with additional properties
          (Girish Palya)

This patch extends the 'completepopup' option with additional settings
to allow more configuration of info popup window.

New values:
```
- close           "on" (default) or "off"
- resize          "on" (default) or "off"
- borderchars     specify eight characters (separated by semicolons) to
                  draw the popup border: top, right, bottom, left,
                  topleft, topright, botright, botleft.
- borderhighlight highlight group for the popup border characters
- shadow          pum shadow
```

closes: #18487

Signed-off-by: Girish Palya <girishji@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>