feat: floating input (#721)

* feat: add floating input to ask method

Open a floating input similar to the "edit" input for the "ask" input.
Enabled in config via `Config.windows.ask.floating` or by passing
`{ floating = true }` to the `api.ask` method.

Includes logic to ensure the sidebar uses the correct buffer and selection
if an existing sidebar is open for another code buffer.

Also refactored the `selection` module to extract the floating input
logic into a new `PromptInput` class.

* docs: update config options

* feat: more accurate annotations to prevent user misunderstandings

---------

Co-authored-by: yetone <yetoneful@gmail.com>

lua/avante/config.lua

@@ -172,6 +172,12 @@ Respect and use existing conventions, libraries, etc that are already present in
     },
     edit = {
       border = "rounded",
+      start_insert = true, -- Start insert mode when opening the edit window
+    },
+    ask = {
+      floating = false, -- Open the 'AvanteAsk' prompt in a floating window
+      border = "rounded",
+      start_insert = true, -- Start insert mode when opening the ask window
     },
   },
   --- @class AvanteConflictConfig