Add onDuplicateKeyUpdate() support to Insert class

[jerryxh]

Feature Request
MySQL/MariaDB supports INSERT ... ON DUPLICATE KEY UPDATE for upsert operations, but Atlas doesn't currently provide a way to generate this syntax.

Proposed API
Add an onDuplicateKeyUpdate() method to the Insert class:

  /**
   * Add ON DUPLICATE KEY UPDATE clause to INSERT statement
   *
   * @param array $columns Column names and values to update on duplicate key
   * @return static
   */
  public function onDuplicateKeyUpdate(array $columns): static

Example Usage

  $insert = Insert::new('mysql')
      ->into('users')
      ->columns(['id' => 1, 'name' => 'John'])
      ->onDuplicateKeyUpdate(['name' => 'Jane']);

  echo $insert->getQueryString();
  // INSERT INTO users (id, name) VALUES (?, ?) ON DUPLICATE KEY UPDATE name = ?

  print_r($insert->getBindValueArrays());
  // [1, 'John', 'Jane']

Use Case
This prevents race condition errors when multiple processes might insert the same record simultaneously -- the first succeeds with INSERT, subsequent ones UPDATE instead of throwing duplicate key errors.

Is this something you'd accept?