refactor!: scope lsp options under elixirls key

[mhanberg]

When setting up elixir-ls, one should now scope all of the opts under an elixirls key in the opts table.

Old:

local elixir = require("elixir")

elixir.setup {
  settings = {
    -- some settings
  }
}

New:

local elixir = require("elixir")

elixir.setup {
  elixirls = {
    settings = {
      -- some settings
    }
  }
}

In addition, the function to merge settings into the default settings now lives in the elixirls module

Old:

local elixir = require("elixir")

elixir.settings {
  -- some settings
}

New:

local elixirls = require("elixir.elixirls")

elixirls.settings {
  -- some settings
}