-> Work done by @traxys and myself

The current maps option which allows to define keymaps in nixvim has a few flaws.
For instance, it is not possible to declare a mapping in several modes at once.

In order to wrap the underlying vim.keymap.set() function more natively, we have decided to change this fundamental option.
We have introduced a new option called keymaps which expects a list of submodule.

As an example is worth 1000 words, here is what it looks like:

# old syntax:
maps = {
  normalVisualOp."<C-m>" = ":!make<CR>";
  normal."<leader>p" = {
    action = "require('my-plugin').do_stuff";
    lua = true;
    silent = true;
    desc = "My plugin does stuff";
  };
};

# new syntax:
keymaps = [
  {
    # Default mode is "" which means normal-visual-op
    key = "<C-m>";
    action = ":!make<CR>";
  }
  {
    # Mode can be a string or a list of strings
    mode = "n";
    key = "<leader>p";
    action = "require('my-plugin').do_stuff";
    lua = true;
    # Note that all of the mapping options are now under the `options` attrs
    options = {
      silent = true;
      desc = "My plugin does stuff";
    };
  }
];

We would like to highlight that this new syntax feels way more natural regarding the neovim API.
This is the main motivation for this change as well as the numerous simplifications it thus brings.

Note: For now, the legacy maps option is still supported and will cause a warning to be shown at build time.

Relevant discussion: #576