Fix LSP and update plugins

- Structure LSP configuration code
- Since `lsp-zero` is deprecated, instead use `nvim-lspconfig`
- Use `nvim_lsp_signature_help` instead of `lsp_signature`

nvim/lua/lsp/cmp.lua

@@ -0,0 +1,25 @@
+local cmp = require('cmp')
+local sneng = require(snippet_engine)
+
+cmp.setup({
+    snippet = {
+      expand = function(args)
+        sneng.lsp_expand(args.body)
+      end,
+    },
+	window = {
+		completion = cmp.config.window.bordered(),
+		documentation = cmp.config.window.bordered(),
+	},
+	mapping = cmp.mapping.preset.insert({
+        ['<C-p>'] = cmp.mapping.scroll_docs(-4),
+        ['<C-n>'] = cmp.mapping.scroll_docs(4),
+		['<tab>'] = cmp.mapping.confirm({ select = true }),
+		['<C-space>'] = cmp.mapping.complete(),
+    }),
+    sources = cmp.config.sources({
+        { name = 'nvim_lsp' },
+        { name = 'nvim_lsp_signature_help' },
+        { name = sneng_name },
+    })
+})