Make object wrapper extend DelayedInit

[MaciejG604]

Fixes #2470

Thanks to @rochala for suggesting the solution of using a wrapper that extends App.

This simple fix seems to have been overlooked in the past, probably because it has some quirks when it comes to different Scala versions.

For Scala 2.12 and 2.13 an object that extends App avoids the deadlock when background threads are started from its initialization clause.
However, this is not true for Scala 3, it is a known bug that has not been fixed yet - scala/scala3#16149

Object wrapper extending DelayedInit can be chosen with --delayed-init and //> using delayedInit.wrapper.
This wrapper will not suffer from deadlocks in Scala 2.13 and 2.12 when background threads are used.

Since DelayedInit is deprecated, this mechanism won't be the default.
Using App instead of DelayedInit requires a bit more work as it introduces an extra main function, which messes up with Scala CLI's main class inference from scripts.

[Gedochao]

LGTM

[som-snytt]

This simple fix seems to have been overlooked in the past

Previous REPL wrap is App scala/scala#4246

Uncharacteristically, I submitted it three times. I don't remember why it did not fly.

[MaciejG604]

May be replaced by #2556

[MaciejG604]

Replaced by #2556