fix: set the popup window to markdown filetype

Author: XXiaoA

lua/gp/init.lua

@@ -1573,6 +1573,8 @@ M.open_buf = function(file_name, target, kind, toggle)
 			-- read file into buffer and force write it
 			vim.api.nvim_command("silent 0read " .. file_name)
 			vim.api.nvim_command("silent file " .. file_name)
+			-- set the filetype to markdown
+			vim.api.nvim_buf_set_option(buf, "filetype", "markdown")
 		else
 			-- move cursor to the beginning of the file and scroll to the end
 			M._H.feedkeys("ggG", "xn")