fix(ui): 'pumborder' parsing, shadow #36127

Problem:
1. Setting `pumborder=+,+,+,+,+,+,+,+` failed to render the custom
   border characters correctly. The issue occurred in `parse_winborder()`
   where it incorrectly used `p_winborder` instead of the `border_opt`
   parameter when the option value didn't contain commas.
2. In `pum_redraw()`, calling `parse_border_style()` directly with the
   option string failed to parse comma-separated border characters.
3. Missing documentation for PmenuShadow and PmenuShadowThrough
   highlight groups used by the shadow border style.
4. Coverity reports CID 631420: passing WinConfig (480 bytes) by value
   in `grid_draw_border()`.
5. crash when using `shadow` value on pumborder.

Solution:
1. Fix `parse_winborder()` to use `border_opt` parameter consistently,
   ensuring the correct option value is parsed regardless of which
   option (winborder/pumborder) is being set.
2. Update `pum_redraw()` to call `parse_winborder()` instead of
   `parse_border_style()`, properly handling both predefined styles
   and custom comma-separated border characters.
3. Add documentation for PmenuShadow (blended shadow areas) and
   PmenuShadowThrough (see-through corners) highlight groups.
4. Change `grid_draw_border()` to accept WinConfig by pointer.
5. When the "shadow" style is used, no additional row and column offset
   is applied, and the border width is reduced.

runtime/lua/vim/_meta/options.lua

@@ -5130,7 +5130,8 @@ vim.go.pumblend = vim.o.pumblend
 vim.go.pb = vim.go.pumblend
 
 --- Defines the default border style of popupmenu windows. Same as
---- 'winborder'.
+--- 'winborder'. `hl-PmenuBorder` is used. When style is "shadow", the
+--- `hl-PmenuShadow` and `hl-PmenuShadowThrough` are used.
 ---
 --- @type string
 vim.o.pumborder = ""