Add MySQL support for `upsert()`

[aleclarson]

For upserts, MySQL uses different syntax than Postgres or SQLite: https://www.perplexity.ai/search/how-does-mysql-upsert-differ-f-MUag9aX5Squ0y8pg0zm.bg

Note that Drizzle doesn't provide a returning() method for MySQL, though it does provide $returningId().

If you're interested in working on this, you may fork the wip/mysql-upsert branch.