feat: support windows position (#436)

lua/avante/config.lua

@@ -18,8 +18,6 @@ M.defaults = {
   -- For most providers that we support we will determine this automatically.
   -- If you wish to use a given implementation, then you can override it here.
   tokenizer = "tiktoken",
-  ---@type "vertical" | "horizontal"
-  layout = "vertical",
   ---@type AvanteSupportedProvider
   openai = {
     endpoint = "https://api.openai.com/v1",
@@ -137,6 +135,8 @@ M.defaults = {
     },
   },
   windows = {
+    ---@type "right" | "left" | "top" | "bottom"
+    position = "right",
     wrap = true, -- similar to vim.o.wrap
     width = 30, -- default % based on available width in vertical layout
     height = 30, -- default % based on available height in horizontal layout