feat: add nestMode option for managed transactions

[ephys]

Pull Request Checklist

• Have you added new tests to prevent regressions?
• If a documentation update is necessary, have you opened a PR to the documentation repository?
• Did you update the typescript typings accordingly (if applicable)?
• Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
• Does the name of your PR follow our conventions?

Description Of Change

This PR implements the feature described here: #13741 (comment), except for withAlsTransaction which we can always add if there is an actual need for it

Breaking changes

• Normalized the casing of our transaction-related enums. It's only a typing change, because we never actually exposed the value as a global export.
  • LOCK -> Lock
  • ISOLATION_LEVELS -> TransactionType
  • TRANSACTION_TYPES -> TransactionType

[WikiRik]

One thing you forgot to update but apart from that it's good