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.go

@@ -0,0 +1,20 @@
+package validate
+
+import (
+	"regexp"
+)
+
+var (
+	ErrNoMatch = NewError("invalid format")
+)
+
+// Match validates that a string matches a regular expression. Compile the
+// expression once with regexp.MustCompile and reuse the returned validator.
+func Match(re *regexp.Regexp) func(string) error {
+	return func(value string) error {
+		if !re.MatchString(value) {
+			return ErrNoMatch
+		}
+		return nil
+	}
+}