Select which version to see compiler options

[som-snytt]

Per https://github.com/scala/docs.scala-lang/issues/1711#issuecomment-628347181

If compiler command line options aka settings are worth documenting, it would be useful to be able to select the version of interest.

This may be more urgent for Scala 3.

[SethTisue]

perhaps someone at the Scala Center (or engaged by the Center) has a plan for this site-wide?

for 2.12 vs 2.13 collections, we special-cased it rather than devise a mechanism

[som-snytt]

Some people are married to their work, others are only engaged.

[sjrd]

We haven't really thought through the version-dependent documentation thing yet, I'm afraid.

[som-snytt]

Sample unification observation at https://github.com/scala/docs.scala-lang/issues/2352