Allow macro to become configurable via compiler settings (similar to -Xmacro-settings in Scala 2)

[olafurpg]

Currently, it's possible for blackbox Scala 2 macros to access compiler settings via -Xmacro-settings:hello-world

def impl(c: Context): c.Tree =
  c.settings // List("hello-world")

I wasn't able to find the equivalent functionality while writing a Scala 3 macro. This feature would be helpful to make the behavior of a macro configurable per-project.