feat(pack): allow skip install confirmation in add()

Problem: No way to skip install confirmation in `add()`. Having install
  confirmation by default is a more secure design. However, users are
  usually aware of the fact that plugin will be installed and there is
  currently no way to skip confirmation.

  Plus it can introduce inconvenience on the clean config initialization
  if it is modularized with many `vim.pack.add()` calls (leads to
  confirming installation many times in a row).

Solution: Add `opts.confirm` option that can skip install confirmation.

test/functional/plugin/pack_spec.lua

@@ -332,6 +332,22 @@ describe('vim.pack', function()
       eq({ confirm_msg, 'Proceed? &Yes\n&No', 1, 'Question' }, exec_lua('return _G.confirm_args'))
     end)
 
+    it('respects `opts.confirm`', function()
+      exec_lua(function()
+        _G.confirm_used = false
+        ---@diagnostic disable-next-line: duplicate-set-field
+        vim.fn.confirm = function()
+          _G.confirm_used = true
+          return 1
+        end
+
+        vim.pack.add({ repos_src.basic }, { confirm = false })
+      end)
+
+      eq(false, exec_lua('return _G.confirm_used'))
+      eq('basic main', exec_lua('return require("basic")'))
+    end)
+
     it('installs at proper version', function()
       local out = exec_lua(function()
         vim.pack.add({