feat(api): nvim_exec2(), deprecate nvim_exec() #19032

Problem:
The signature of nvim_exec() is not extensible per ":help api-contract".

Solution:
Introduce nvim_exec2() and deprecate nvim_exec().

runtime/lua/_vim9script.lua

@@ -162,7 +162,7 @@ local vim9 = (function()
           end
         end
 
-        vim.api.nvim_exec(table.concat(file, '\n'), false)
+        vim.api.nvim_exec2(table.concat(file, '\n'), { output = false })
       end,
     })
   end