feat: show keymaps for builtin actions (#1084)

* Add default mappings `<C-/>`and `?` for insert and normal mode, respectively, to show registered keymappings (`actions.which_key`) attached to prompt buffer

lua/telescope/actions/utils.lua

@@ -78,4 +78,26 @@ function utils.map_selections(prompt_bufnr, f)
   end
 end
 
+local findnth = function(str, nth)
+  local array = {}
+  for i in string.gmatch(str, "%d+") do
+    table.insert(array, tonumber(i))
+  end
+  return array[nth]
+end
+
+--- Utility to collect mappings of prompt buffer in array of `{mode, keybind, name}`.
+---@param prompt_bufnr number: The prompt bufnr
+function utils.get_registered_mappings(prompt_bufnr)
+  local ret = {}
+  for _, mode in ipairs { "n", "i" } do
+    local mode_mappings = vim.api.nvim_buf_get_keymap(prompt_bufnr, mode)
+    for _, mapping in ipairs(mode_mappings) do
+      local funcid = findnth(mapping.rhs, 2)
+      table.insert(ret, { mode = mode, keybind = mapping.lhs, func = __TelescopeKeymapStore[prompt_bufnr][funcid] })
+    end
+  end
+  return ret
+end
+
 return utils