Enhance help message for language flag

[noti0na1]

Partial fix #20083

Enhance help message for language flag by displaying all available choises

> scalac -language:help
-language  Enable one or more language features.
           Choices : 
           - help :
                Display all available features, 
           - noAutoTupling :
                Disable automatic tupling, 
           - dynamics :
                Allow direct or indirect subclasses of scala.Dynamic, 
           - unsafeNulls :
                Enable unsafe nulls for explicit nulls, 
           - postfixOps :
                Allow postfix operators (not recommended), 
           - strictEquality :
                Enable strict equality (disable canEqualAny), 
           - implicitConversions :
                Allow implicit conversions without warnings, 
           - adhocExtensions :
                Allow ad-hoc extension methods, 
           - experimental.namedTypeArguments :
                Allow named type arguments, 
           - experimental.genericNumberLiterals :
                Allow generic number literals, 
           - experimental.macros :
                Allow Scala 2 macros, 
           - experimental.dependent :
                Allow dependent method types, 
           - experimental.erasedDefinitions :
                Allow erased definitions, 
           - deprecated.symbolLiterals :
                Allow symbol literals, 
           - experimental.fewerBraces :
                Enable support for using indentation for arguments, 
           - experimental.saferExceptions :
                Enable safer exceptions, 
           - experimental.clauseInterleaving :
                Enable clause interleaving, 
           - experimental.pureFunctions :
                Enable pure functions for capture checking, 
           - experimental.captureChecking :
                Enable experimental capture checking, 
           - experimental.into :
                Allow into modifier on parameter types

[Gedochao]

@noti0na1 perhaps we could align the output with the way Scala 2 compiler does it?

scalac2 -language:help # scalac2 is an alias for the Scala 2 compiler command here
# Enable or disable language features
#   dynamics             Allow direct or indirect subclasses of scala.Dynamic
#   existentials         Existential types (besides wildcard types) can be written and inferred
#   higherKinds          Allow higher-kinded types
#   implicitConversions  Allow definition of implicit functions called views
#   postfixOps           Allow postfix operator notation, such as `1 to 10 toList` (not recommended)
#   reflectiveCalls      Allow reflective access to members of structural types
#   experimental.macros  Allow macro definition (besides implementation and application)

Also, optimally we'd like passing inputs to not be necessary with a help flag (so no empty.scala should be necessary)

[odersky]

I am not sure what the changes in the CB build files are for. The changes in config LGTM.

[noti0na1]

I am not sure what the changes in the CB build files are for.

@odersky Since this change will reject invalid language features, I just delete old flags (like higherKind from Scala 2) in CB.

[WojciechMazur]

I wonder would it be possible to make the unknown -language options yield warning instead of error. This way it would be more alligned with handling of other unknown scalac options - these are ignored are yield warning. I've found it quite annoying when reproducing OpenCB failures when projects use plugins to set common scalacOptions - these typically might contain deprecated or removed in Scala 3 scalac options.
#20419