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.

runtime/doc/pack.txt

@@ -341,6 +341,8 @@ add({specs}, {opts})                                          *vim.pack.add()*
                    works like `:packadd!`. If function, called with plugin
                    data and is fully responsible for loading plugin. Default
                    `false` during startup and `true` afterwards.
+                 • {confirm}? (`boolean`) Whether to ask user to confirm
+                   initial install. Default `true`.
 
 del({names})                                                  *vim.pack.del()*
     Remove plugins from disk

runtime/lua/vim/pack.lua

@@ -562,9 +562,9 @@ local function checkout(p, timestamp, skip_same_sha)
 end
 
 --- @param plug_list vim.pack.Plug[]
-local function install_list(plug_list)
+local function install_list(plug_list, confirm)
   -- Get user confirmation to install plugins
-  if not confirm_install(plug_list) then
+  if confirm and not confirm_install(plug_list) then
     for _, p in ipairs(plug_list) do
       p.info.err = 'Installation was not confirmed'
     end
@@ -681,6 +681,8 @@ end
 --- If function, called with plugin data and is fully responsible for loading plugin.
 --- Default `false` during startup and `true` afterwards.
 --- @field load? boolean|fun(plug_data: {spec: vim.pack.Spec, path: string})
+---
+--- @field confirm? boolean Whether to ask user to confirm initial install. Default `true`.
 
 --- Add plugin to current session
 ---
@@ -705,7 +707,7 @@ end
 --- @param opts? vim.pack.keyset.add
 function M.add(specs, opts)
   vim.validate('specs', specs, vim.islist, false, 'list')
-  opts = vim.tbl_extend('force', { load = vim.v.vim_did_enter == 1 }, opts or {})
+  opts = vim.tbl_extend('force', { load = vim.v.vim_did_enter == 1, confirm = true }, opts or {})
   vim.validate('opts', opts, 'table')
 
   --- @type vim.pack.Plug[]
@@ -720,7 +722,7 @@ function M.add(specs, opts)
 
   if #plugs_to_install > 0 then
     git_ensure_exec()
-    install_list(plugs_to_install)
+    install_list(plugs_to_install, opts.confirm)
   end
 
   -- Register and load those actually on disk while collecting errors

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({