add Required, Prefix, Suffix, Contains, Match validators

Broadens the built-in set with the string/presence checks that came up
most often when reaching for this package:

  - Required[T comparable]  non-zero value
  - Prefix / Suffix / Contains  strings.HasPrefix/HasSuffix/Contains
  - Match(*regexp.Regexp)  arbitrary pattern

All follow the existing shape: a constructor returning func(T) error, a
sentinel error matched with errors.Is, table-driven tests and an Example.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

required.go

@@ -0,0 +1,21 @@
+package validate
+
+var (
+	ErrRequired = NewError("is required")
+)
+
+// Required validates that a value is not the zero value for its type: an
+// empty string, 0, a nil pointer, and so on.
+//
+// It is defined for comparable types only. For slices and maps, use
+// MinSize(1); for "non-whitespace", combine with a trimming step.
+func Required[T comparable]() func(T) error {
+	var zero T
+
+	return func(value T) error {
+		if value == zero {
+			return ErrRequired
+		}
+		return nil
+	}
+}