Restructure Darwin configuration

So far, this is just based on mightymac. The other devices will be
updated soon.

modules/home-manager/files/nvim/lua/plugins/completions.lua

@@ -1,49 +0,0 @@
-return {
-  {
-    "hrsh7th/nvim-cmp",
-    config = function()
-      local cmp = require("cmp")
-      require("luasnip.loaders.from_vscode").lazy_load()
-
-      cmp.setup({
-        snippet = {
-          expand = function(args)
-            require("luasnip").lsp_expand(args.body)
-          end,
-        },
-        window = {
-          completion = cmp.config.window.bordered(),
-          documentation = cmp.config.window.bordered(),
-        },
-        mapping = cmp.mapping.preset.insert({
-          ["<C-b>"] = cmp.mapping.scroll_docs(-4),
-          ["<C-f>"] = cmp.mapping.scroll_docs(4),
-          ["<C-Space>"] = cmp.mapping.complete(),
-          ["<C-e>"] = cmp.mapping.abort(),
-          ["<CR>"] = cmp.mapping.confirm({ select = true }),
-        }),
-        sources = cmp.config.sources({
-          {
-            name = "nvim_lsp",
-            entry_filter = function(entry)
-              return require("cmp.types").lsp.CompletionItemKind[entry:get_kind()] ~= "Text"
-            end,
-          },
-          { name = "luasnip" },
-        }, {
-          { name = "buffer" },
-        }),
-      })
-    end,
-  },
-  {
-    "hrsh7th/cmp-nvim-lsp",
-  },
-  {
-    "L3MON4D3/LuaSnip",
-    dependencies = {
-      "saadparwaiz1/cmp_luasnip",
-      "rafamadriz/friendly-snippets",
-    },
-  },
-}