feat(float): add winborder option (#31074)

Problem:
There is currently no global option to define the default border style for floating windows. This leads to repetitive code when developers need consistent styling across multiple floating windows.

Solution:
Introduce a global option winborder to specify the default border style for floating windows. When a floating window is created without explicitly specifying a border style, the value of the winborder option will be used. This simplifies configuration and ensures consistency in floating window appearance.

Co-authored-by: Gregory Anders <greg@gpanders.com>

src/nvim/options.lua

@@ -10188,6 +10188,26 @@ local options = {
       short_desc = N_('Controls transparency level for floating windows'),
       type = 'number',
     },
+    {
+      defaults = { if_true = '' },
+      cb = 'did_set_winborder',
+      values = { '', 'double', 'single', 'shadow', 'rounded', 'solid', 'none' },
+      desc = [=[
+        Defines the default border style of floating windows. The default value
+        is empty, which is equivalent to "none". Valid values include:
+        - "none": No border.
+        - "single": A single line box.
+        - "double": A double line box.
+        - "rounded": Like "single", but with rounded corners ("╭" etc.).
+        - "solid": Adds padding by a single whitespace cell.
+        - "shadow": A drop shadow effect by blending with the background.
+      ]=],
+      full_name = 'winborder',
+      scope = { 'global' },
+      short_desc = N_('border of floating window'),
+      type = 'string',
+      varname = 'p_winbd',
+    },
     {
       abbreviation = 'wi',
       cb = 'did_set_window',