Publish scala2-library-tasty-experimental (#19588)

We will start publishing
`org.scala-lang::scala2-library-tasty-experimental`. This is __not
considered fully stable__.

We publish it to allow users to depend on it for __stabilization
purposes__. Once this version of the library is considered fully stable
it will become a default dependency of any Scala build.

Example usage in SBT
```scala
libraryDependencies += "org.scala-lang" %% "scala2-library-tasty-experimental" % scalaVersion.value
```

### Behaviour changes
In general, nothing should when compiling against this version of the
standard library. If something changes it should be flagged. But this
version of the library has more information about the code, which can
lead to new warnings (and maybe errors).
* Better pure expression detection, for example: pure trait
initialization
* More information for the initialization checker. It might catch more
errors that involve the initialization of library classes.

project/Build.scala

@@ -2121,7 +2121,7 @@ object Build {
     // FIXME: we do not aggregate `bin` because its tests delete jars, thus breaking other tests
     def asDottyRoot(implicit mode: Mode): Project = project.withCommonSettings.
       aggregate(`scala3-interfaces`, dottyLibrary, dottyCompiler, tastyCore, `scala3-sbt-bridge`, scala3PresentationCompiler).
-      bootstrappedAggregate(`scala3-language-server`, `scala3-staging`,
+      bootstrappedAggregate(`scala2-library-tasty`, `scala3-language-server`, `scala3-staging`,
         `scala3-tasty-inspector`, `scala3-library-bootstrappedJS`, scaladoc).
       dependsOn(tastyCore).
       dependsOn(dottyCompiler).