feat: add keymaps option for scrolling the hover window

[zuoxinyu]

Description

Add keymaps for scrolling the hover window without entering it (e.g. by press shift-K twice to enter it), just like the behavior in nvim-cmp. It's disabled by default, cause <c-j>/<c-k> might confilict with users' keymappings.

• Add an option in the config to enable and custom the keymaps

  -- keymaps for scrolling the hover window
  keymaps = {
    -- enable the keymaps
    -- default: false
    enable = false,
    -- scroll up
    -- default: "<c-j>"
    scroll_up = "<c-j>",
    -- scroll down
    -- default: "<c-k>"
    scroll_down = "<c-k>",
  },

• Add and expose a method scroll_hover(number) for the hover_actions module.