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>

string_test.go

@@ -0,0 +1,69 @@
+package validate
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+func ExamplePrefix() {
+	fmt.Println(Prefix("https://")("http://example.com"))
+	// Output: must start with "https://"
+}
+
+func TestPrefix(t *testing.T) {
+	testCases := map[string]error{
+		"https://example.com": nil,
+		"https://":            nil,
+
+		"http://example.com": ErrMissingPrefix.With("https://"),
+		"":                   ErrMissingPrefix.With("https://"),
+	}
+
+	v := Prefix("https://")
+	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)
+			}
+		})
+	}
+}
+
+func TestSuffix(t *testing.T) {
+	testCases := map[string]error{
+		"report.pdf": nil,
+		".pdf":       nil,
+
+		"report.txt": ErrMissingSuffix.With(".pdf"),
+		"pdf":        ErrMissingSuffix.With(".pdf"),
+	}
+
+	v := Suffix(".pdf")
+	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)
+			}
+		})
+	}
+}
+
+func TestContains(t *testing.T) {
+	testCases := map[string]error{
+		"a b c": nil,
+		" ":     nil,
+
+		"abc": ErrMissingSubstring.With(" "),
+		"":    ErrMissingSubstring.With(" "),
+	}
+
+	v := Contains(" ")
+	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)
+			}
+		})
+	}
+}