Add insertOrUpdate() for upsert operations

[dereuromark]

Summary

I realized that some interface changes might be necessary, so I thought better tackle this before we tag 5.x stable.

Implements insertOrUpdate() method for upsert operations - inserting new rows and updating existing rows on duplicate key conflicts.

Resolves #950

Features

• New insertOrUpdate() method on Table class and seed interface
• Requires explicit conflict columns (columns that define uniqueness)
• Requires explicit update columns (columns to update on conflict)
• Works with both single row and bulk inserts

Database Support

Database	Syntax
MySQL	ON DUPLICATE KEY UPDATE col = VALUES(col)
PostgreSQL	ON CONFLICT (cols) DO UPDATE SET col = EXCLUDED.col
SQLite	ON CONFLICT (cols) DO UPDATE SET col = excluded.col

SQL Server support is not included in this PR.

Usage

// In migrations or seeds
$table->insertOrUpdate([
    ['code' => 'USD', 'rate' => 1.0000],
    ['code' => 'EUR', 'rate' => 0.9234],
], ['rate'], ['code'])->save();

// In seeds (shorthand)
$this->insertOrUpdate('currencies', [
    ['code' => 'USD', 'rate' => 1.0000],
    ['code' => 'EUR', 'rate' => 0.9234],
], ['rate'], ['code']);