Standalone using directives library

[tgodzik]

Is your feature request related to a problem? Please describe. During the last tooling summit we discussed how multiple tools could support and reuse the using directives. This led us to a conclusion that we need a separate directives library similar to @alexarchambault efforts on that front and continuing his work.

Describe the solution you'd like

Main points of that library would be:

• it should we have scala native and js, Scala 3 and Scala 2.13
• the most common directives should be implemented right from the start, most likely anything that is not behind --power in Scala CLI. This doesn't mean each tool will have to support them, they can still decide to ignore them or report an error. So ideally we would define a parser and the rules it should follow, very roughly:
• it should be easy for any tool to add their own directives such as ammonite.version or all the directives behind --power in Scala CLI
• defaults should not have aliases, but the tool might want to add them to preserve compatibility (most likely Scala CLI) -we should allow some basic types and parsing errors

Very very roughly:

class Parser (rules: Directive*) { def parse(path: Path): List[DirectiveValue] = ??? }
class Directive[T](name: String, helpMessage: String, validation: T => Boolean)
class DirectiveValue[T](origin: Directive[T], value: T)
val parser = new Parser(
  StandardDirectives.scalaVersion,
  StandardDirectives.dependency.withAliases("dep", "lib"),
  new Directive[String]("ammonite.version", "help", validationMethod = (value: String) => value.contains(".") )
)
values = parser.parse(Paths.get("./Hello.scala"))

Additionally:

• directives should have positions, these can be used for automatic fixes etc.
• we should warn about directives which are not known (or error, this could be a setting of the parser)

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered.

Bonus point No macros.