Add support for grouped keys

[musjj]

If you have a lot of keys that you want to use for lazy loading, your config can become a bit hairy:

use {
  "FooBar/Baz",
  keys = {
    { "n", "a" },
    { "n", "b" },
    { "n", "c" },
    { "n", "d" },
    { "v", "e" },
    { "v", "f" },
    { "v", "g" },
    { "v", "h" },
  },
}

With this PR, you can now instead group your keys like this:

use {
  "FooBar/Baz",
  keys = {
    n = { "a", "b", "c", "d" },
    v = { "e", "f", "g", "h" },
  },
}

[musjj]

Anything blocking this from getting merged?
To clarify, this PR is backwards-compatible with the original way of specifying keys.

[wbthomason]

Thanks! Sorry for not seeing this before now. I think this is mostly good to go, but the updated language in the docs is technically incorrect, so we need to fix that before merging.

I'll have limited internet for the next week, but please @ me if you get to this and I haven't responded by Dec 21.

[wbthomason]

Here and in the doc/ file, we need to change the wording a bit. It currently only talks about individual keybindings, but we want to say that you can specify the whole set of keybindings in one of these two table formats, not individual keybindings.

[musjj]

Sorry for the late reply! Do you think they're clearer now?

[XXiaoA]

Hey @musjj! thanks for your great PR, which I really need. Therefore, when will we fix the docs?🤗