feat(nvim-cmp): Add tab complete when there is no ambiguity

- from <https://github.com/hrsh7th/nvim-cmp/wiki/Example-mappings#confirm-candidate-on-tab-immediately-when-theres-only-one-completion-entry>

lua/complete.lua

@@ -9,6 +9,12 @@ luasnip.config.setup {
   enable_autosnippets = true,
 }
 
+local has_words_before = function()
+  unpack = unpack or table.unpack
+  local line, col = unpack(vim.api.nvim_win_get_cursor(0))
+  return col ~= 0 and vim.api.nvim_buf_get_lines(0, line - 1, line, true)[1]:sub(col, col):match("%s") == nil
+end
+
 cmp.setup {
   snippet = {
     expand = function(args)
@@ -32,9 +38,18 @@ cmp.setup {
     ['<C-e>'] = cmp.mapping({ i = cmp.mapping.abort(), c = cmp.mapping.close() }),
     ['<Tab>'] = cmp.mapping(function(fallback)
       if cmp.visible() then
-        cmp.select_next_item()
+        if #cmp.get_entries() == 1 then
+          cmp.confirm({select = true})
+        else
+          cmp.select_next_item()
+        end
       elseif luasnip.expand_or_locally_jumpable() then
         luasnip.expand_or_jump()
+      elseif has_words_before() then
+        cmp.complete()
+        if #cmp.get_entries() == 1 then
+          cmp.confirm({select = true})
+        end
       else
         fallback()
       end