feat: add toBeEmail expectation (#1735)

Adds `toBeEmail()` backed by `filter_var(FILTER_VALIDATE_EMAIL)`.
Mirrors the existing `toBeUrl()` pattern — no new dependencies, pure PHP.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Support/Str.php

@@ -118,6 +118,14 @@ final class Str
         return sprintf(str_repeat('`%s` → ', count($describeDescriptions)).'%s', ...$descriptionComponents);
     }
 
+    /**
+     * Determine if a given value is a valid email address.
+     */
+    public static function isEmail(string $value): bool
+    {
+        return (bool) filter_var($value, FILTER_VALIDATE_EMAIL);
+    }
+
     /**
      * Determine if a given value is a valid URL.
      */