feat(lsp): add helper function for getting server settings

[mehalter]

This further improves the user experience for setting up LSP specific plugins. With this new helper function the user can do the following:

return {
  lsp = {
    skip_setup = { "tsserver" },
  },
  plugins = {
    init = {
      {
        "jose-elias-alvarez/typescript.nvim",
        after = "nvim-lsp-installer",
        config = function()
          require("typescript").setup {
            server = astronvim.lsp.server_settings("tsserver")
          }
        end,
      },
    },
    ["nvim-lsp-installer"] = {
      ensure_installed = { "tsserver" },
    },
  },
}