Add source options

[mihaisoloi]

The docs are lifted from here:

• https://docs.scala-lang.org/scala3/guides/migration/tooling-tour.html#the-scala-213-compiler
• https://docs.scala-lang.org/scala3/reference/language-versions/source-compatibility.html

These are not enabled by default as they add a bunch of warnings even if the code is completely valid in the current Scala version.

[DavidGregory084]

Thanks for this @mihaisoloi - I have actually been sitting on a PR that implements these flags because I couldn't think of a nice way to resolve the 2.x -Xsource vs 3.x -source inconsistency, so in a way it's a relief to see that you also went for scala3Source!
I'd like to compare what you have done here to my branch and merge the changes together since I went spelunking into the history of the ScalaSettings code in scala/scala and lampepfl/dotty to figure out when each flag was added.

[DavidGregory084]

Thanks @mihaisoloi! This is superseded by #64