feat!(selection): do not display hint immediately

Selection hint that is displayed immediately upon entering visual mode
([<leader>aa: ask, <leader>ae: edit]) gets old pretty quickly. Add a
config option to control when the hint is displayed:

  selection = {
    enabled = true,
    hint_display = "delayed",
  },

The "hint_display" option recognizes the following values:

- "immediate" results in the hint being shown immediately after entering
  visual mode. This is the old behavior.
- "delayed" causes the hint be displayed only if the cursor has not been
  moved for vim.o.updatetime milliseconds. This is the new default.
- "none" suppresses showing the hint completely.

Unfortunately "CursorHold" event is not emitted in visual mode so we
have to emulate it using Utils.debounce().

This is a breaking change because selection behavior was controller by
"hints" config entry which makes little sense, so the config section and
associated commands were renamed to "selection".

Additionally the "hints"/"selection" was mapped to "<leader>ah", but the
very same key combination was used to select from old Avante chat
histories, which overrode the toggle. New selection toggle keymap is
"<leader>aC".

README.md

@@ -452,7 +452,10 @@ _See [config.lua#L9](./lua/avante/config.lua) for the full config_
       close_from_input = nil, -- e.g., { normal = "<Esc>", insert = "<C-d>" }
     },
   },
-  hints = { enabled = true },
+  selection = {
+    enabled = true,
+    hint_display = "delayed",
+  },
   windows = {
     ---@type "right" | "left" | "top" | "bottom"
     position = "right", -- the position of the sidebar