Feature: Add xand tag (#442)

* Feat: Add xand group and check for missing

* Fix: Split and combine err in TestMultiand for consistency

* Feat: Check missing required flags in xand groups

* Feat: Handle combined xor and xand

* Docs: Add info about combined xand and required use

* Docs: Fix language error in xand description

Co-authored-by: Stautis <thkrst@gmail.com>

* Feat: Rename xand to and

* Refactor: Switch from fmt.Sprintf to err.Error

* Refactor: Get requiredAndGroup map in separate function

---------

Co-authored-by: Stautis <thkrst@gmail.com>

build.go

@@ -323,6 +323,7 @@ func buildField(k *Kong, node *Node, v reflect.Value, ft reflect.StructField, fv
 			Envs:        tag.Envs,
 			Group:       buildGroupForKey(k, tag.Group),
 			Xor:         tag.Xor,
+			And:         tag.And,
 			Hidden:      tag.Hidden,
 		}
 		value.Flag = flag