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>

match_test.go

@@ -0,0 +1,35 @@
+package validate
+
+import (
+	"errors"
+	"fmt"
+	"regexp"
+	"testing"
+)
+
+func ExampleMatch() {
+	slug := Match(regexp.MustCompile(`^[a-z0-9-]+$`))
+	fmt.Println(slug("Not A Slug"))
+	// Output: invalid format
+}
+
+func TestMatch(t *testing.T) {
+	v := Match(regexp.MustCompile(`^[a-z0-9-]+$`))
+
+	testCases := map[string]error{
+		"hello-world-123": nil,
+		"abc":             nil,
+
+		"Hello":     ErrNoMatch,
+		"has space": ErrNoMatch,
+		"":          ErrNoMatch,
+	}
+
+	for input, want := range testCases {
+		t.Run(input, func(t *testing.T) {
+			if got := v(input); !errors.Is(got, want) {
+				t.Error("got", got, "want", want)
+			}
+		})
+	}
+}