Refine behavior of -Yno-experimental (#19741)

This is a first step towards the changes described in
#19708 (comment).
This makes testing `-experimental` a bit more robust.

Author: odersky

compiler/src/dotty/tools/dotc/config/Feature.scala

@@ -163,7 +163,7 @@ object Feature:
     do checkExperimentalFeature(s"feature $setting", NoSourcePosition)
 
   def isExperimentalEnabled(using Context): Boolean =
-    (Properties.experimental || ctx.settings.experimental.value) && !ctx.settings.YnoExperimental.value
+    (Properties.experimental && !ctx.settings.YnoExperimental.value) || ctx.settings.experimental.value
 
   /** Handle language import `import language.<prefix>.<imported>` if it is one
    *  of the global imports `pureFunctions` or `captureChecking`. In this case

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -397,7 +397,7 @@ private sealed trait YSettings:
   val YretainTrees: Setting[Boolean] = BooleanSetting("-Yretain-trees", "Retain trees for top-level classes, accessible from ClassSymbol#tree")
   val YshowTreeIds: Setting[Boolean] = BooleanSetting("-Yshow-tree-ids", "Uniquely tag all tree nodes in debugging output.")
   val YfromTastyIgnoreList: Setting[List[String]] = MultiStringSetting("-Yfrom-tasty-ignore-list", "file", "List of `tasty` files in jar files that will not be loaded when using -from-tasty.")
-  val YnoExperimental: Setting[Boolean] = BooleanSetting("-Yno-experimental", "Disable experimental language features.")
+  val YnoExperimental: Setting[Boolean] = BooleanSetting("-Yno-experimental", "Disable experimental language features by default in NIGHTLY/SNAPSHOT versions of the compiler.")
   val YlegacyLazyVals: Setting[Boolean] = BooleanSetting("-Ylegacy-lazy-vals", "Use legacy (pre 3.3.0) implementation of lazy vals.")
   val YcompileScala2Library: Setting[Boolean] = BooleanSetting("-Ycompile-scala2-library", "Used when compiling the Scala 2 standard library.")
   val YoutputOnlyTasty: Setting[Boolean] = BooleanSetting("-Youtput-only-tasty", "Used to only generate the TASTy file without the classfiles")