Fix method_exist() calls throwing \TypeError exceptions on non-st…

…ring|object

PHP 8 throws `\TypeError` and `\ValueError` exceptions in internal functions (including `method_exists()`) when it encounters an invalid value. For `method_exists`, it expects `string|object`.
See https://php.watch/versions/8.0/internal-function-exceptions

`Assert::methodExists()` and `Assert::methodNotExists()` are updated to make sure the provided argument is string|object before calling `method_exist()` to avoid PHP 8 `\TypeError` exceptions.

src/Assert.php

@@ -1620,7 +1620,7 @@ public static function propertyNotExists($classOrObject, $property, $message = '
      */
     public static function methodExists($classOrObject, $method, $message = '')
     {
-        if (!\method_exists($classOrObject, $method)) {
+        if (!(\is_string($classOrObject) || \is_object($classOrObject)) || !\method_exists($classOrObject, $method)) {
             static::reportInvalidArgument(\sprintf(
                 $message ?: 'Expected the method %s to exist.',
                 static::valueToString($method)
@@ -1640,7 +1640,7 @@ public static function methodExists($classOrObject, $method, $message = '')
      */
     public static function methodNotExists($classOrObject, $method, $message = '')
     {
-        if (\method_exists($classOrObject, $method)) {
+        if ((\is_string($classOrObject) || \is_object($classOrObject)) && \method_exists($classOrObject, $method)) {
             static::reportInvalidArgument(\sprintf(
                 $message ?: 'Expected the method %s to not exist.',
                 static::valueToString($method)