Add an argument to the `dt build` and `dt serve` commands to enable experiments

This is necessary for working on an experimental feature that requires the DevTools server.

Locate the command entry points for `dt build` and `dt serve`, then inspect how experimental features are currently enabled in the DevTools server. Define the new argument's expected behavior from those paths and verify that both commands pass it through correctly. No test file or implementation entry point is named in the issue.

Vom Indexierungsmodell aus dem Issue-Text verfasst.