Allow BooleanSettings to be set with a colon

[KacperFKorban]

e.g. "-explain:true" and "-explain:false" should be allowed

[dwijnand]

LGTM. I did the same in Scala 2 a few years ago to make a -Y flag on by default, but allow it to be disabled. Could you add a test that proves that in (a) -foo:true -foo:false false wins and (b) that emits that "Flag $name set repeatedly" warning.

[dwijnand]

❤️

[som-snytt]

In Scala 2, the facility was required to turn off lints. retronym 2014. But dotty has no grouped settings such as -Xlint or -opt.

scala/scala@55549bf

I'm still dubious about flags that default true, just because the options are "underdocumented" or "hard to find the docs". Apparently not everyone scalac -help | grep something to check defaults. Oh, I guess I don't do that on dotty because it's on stderr. The one I always forget is colorization, scalac -help 2>&1 | grep color.