Change completion engine to a more modern one, add neogit for git

operations

nix/home/programs/nvim/nvim-cmp.lua

@@ -0,0 +1,29 @@
+local cmp = require("cmp")
+
+cmp.setup({
+  snippet = {
+    -- REQUIRED - you must specify a snippet engine
+    expand = function(args)
+      -- vim.fn["vsnip#anonymous"](args.body) -- For `vsnip` users.
+      require('luasnip').lsp_expand(args.body) -- For `luasnip` users.
+      -- require('snippy').expand_snippet(args.body) -- For `snippy` users.
+      -- vim.fn["UltiSnips#Anon"](args.body) -- For `ultisnips` users.
+    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 }), -- Accept currently selected item. Set `select` to `false` to only confirm explicitly selected items.
+  }),
+  sources = cmp.config.sources({
+    { name = 'nvim_lsp' },
+  }, {
+    { name = 'buffer' },
+  })
+})