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".

lua/avante/config.lua

@@ -528,7 +528,7 @@ M._defaults = {
     toggle = {
       default = "<leader>at",
       debug = "<leader>ad",
-      hint = "<leader>ah",
+      selection = "<leader>aC",
       suggestion = "<leader>as",
       repomap = "<leader>aR",
     },
@@ -645,9 +645,14 @@ M._defaults = {
     --- Disable by setting to -1.
     override_timeoutlen = 500,
   },
-  --- @class AvanteHintsConfig
-  hints = {
+  --- Allows selecting code or other data in a buffer and ask LLM questions about it or
+  --- to perform edits/transformations.
+  --- @class AvanteSelectionConfig
+  --- @field enabled boolean
+  --- @field hint_display "delayed" | "immediate" | "none" When to show key map hints.
+  selection = {
     enabled = true,
+    hint_display = "delayed",
   },
   --- @class AvanteRepoMapConfig
   repo_map = {