Popup windows in Vim currently have no transparency support. Users cannot create semi-transparent overlays that allow underlying text to be visible through the popup.

So, add a new opacity option for popup windows that controls transparency level.

### Usage

```vim
" Create a popup with 50% opacity (semi-transparent)
let winid = popup_create('Semi-transparent text', #{
    \ line: 5,
    \ col: 10,
    \ opacity: 50,
    \ })

" Change opacity dynamically
call popup_setoptions(winid, #{opacity: 80})

" Get current opacity value
echo popup_getoptions(winid).opacity

Opacity values

• 0 - Fully transparent (background text fully visible)
• 100 - Fully opaque (default, no transparency)
• 1-99 - Partially transparent

Requirements

• Requires 'termguicolors' to be enabled for the blending effect to work