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>

README.md

@@ -262,6 +262,18 @@ _See [config.lua#L9](./lua/avante/config.lua) for the full config_
       align = "center", -- left, center, right for title
       rounded = true,
     },
+    input = {
+      prefix = "> ",
+    },
+    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
+      start_insert = true, -- Start insert mode when opening the ask window, only effective if floating = true.
+      border = "rounded",
+    },
   },
   highlights = {
     ---@type AvanteConflictHighlights