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

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

model.go

@@ -405,6 +405,7 @@ type Flag struct {
 	*Value
 	Group       *Group // Logical grouping when displaying. May also be used by configuration loaders to group options logically.
 	Xor         []string
+	And         []string
 	PlaceHolder string
 	Envs        []string
 	Aliases     []string