feat(option): custom chars in 'winborder' #33772

Problem: winborder option only supported predefined styles and lacked support for custom border characters.

Solution: implement parsing for comma-separated list format that allows specifying 8 individual border characters (topleft, top, topright, right, botright, bottom, botleft, left).

runtime/doc/news.txt

@@ -229,7 +229,7 @@ OPTIONS
 • 'diffopt' `inline:` configures diff highlighting for changes within a line.
 • 'grepformat' is now a |global-local| option.
 • 'pummaxwidth' sets maximum width for the completion popup menu.
-• 'winborder' "bold" style.
+• 'winborder' "bold" style, custom border style.
 • |g:clipboard| accepts a string name to force any builtin clipboard tool.
 • 'busy' sets a buffer "busy" status. Indicated in the default statusline.
 

runtime/doc/options.txt

@@ -7410,6 +7410,10 @@ A jump table for the options with a short description can be found at |Q_op|.
 	- "shadow": Drop shadow effect, by blending with the background.
 	- "single": Single-line box.
 	- "solid": Adds padding by a single whitespace cell.
+	- custom: comma-separated list of exactly 8 characters in clockwise
+	  order starting from topleft. Example: >lua
+	     vim.o.winborder='+,-,+,|,+,-,+,|'
+<
 
 							*'window'* *'wi'*
 'window' 'wi'		number	(default screen height - 1)

runtime/lua/vim/_meta/options.lua

@@ -8117,8 +8117,15 @@ vim.wo.winbl = vim.wo.winblend
 --- - "shadow": Drop shadow effect, by blending with the background.
 --- - "single": Single-line box.
 --- - "solid": Adds padding by a single whitespace cell.
+--- - custom: comma-separated list of exactly 8 characters in clockwise
+---   order starting from topleft. Example:
 ---
---- @type ''|'double'|'single'|'shadow'|'rounded'|'solid'|'bold'|'none'
+--- ```lua
+---      vim.o.winborder='+,-,+,`,+,-,+,`'
+--- ```
+---
+---
+--- @type string
 vim.o.winborder = ""
 vim.go.winborder = vim.o.winborder