Support `flags` in @@config in playground

[fhammerschmidt]

Currently in the playground this yields an error:

@@config({flags: ["-enable-experimental", "LetUnwrap"]})
// [E] Line 1, column 10:
// flags is not supported

https://rescript-lang.org/try?version=v12.0.0-beta.8&module=esmodule&code=ALDGHsDsDMEsHMAUBvaAbAhvAzgLgAQDaARALQCmkGARmuRQB4AO5ATrALaUAuGaxAGnzEAMuW4BVSAHdWGJsQC6AXwCUAKCA

This is unfortunate for showcasing the new feature quickly. And I am not sure where the culprit is so I am adding this issue to the main repo for now.

[cknitt]

Maybe there should be toggles in the UI instead?

[cknitt]

Same for preserve mode

Maybe have a text field to extend bsc-flags that have no dedicated UI element yet?