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>

tag.go

@@ -32,6 +32,7 @@ type Tag struct {
 	Enum        string
 	Group       string
 	Xor         []string
+	And         []string
 	Vars        Vars
 	Prefix      string // Optional prefix on anonymous structs. All sub-flags will have this prefix.
 	EnvPrefix   string
@@ -249,6 +250,9 @@ func hydrateTag(t *Tag, typ reflect.Type) error { //nolint: gocyclo
 	for _, xor := range t.GetAll("xor") {
 		t.Xor = append(t.Xor, strings.FieldsFunc(xor, tagSplitFn)...)
 	}
+	for _, and := range t.GetAll("and") {
+		t.And = append(t.And, strings.FieldsFunc(and, tagSplitFn)...)
+	}
 	t.Prefix = t.Get("prefix")
 	t.EnvPrefix = t.Get("envprefix")
 	t.Embed = t.Has("embed")