Support using directives in hovers and completions

[tgodzik]

Is your feature request related to a problem? Please describe.

Using directives are not suggested or explained by Metals

Describe the solution you'd like

Metals should:

• suggest using directive keys
• using keyword
• show docs on hover
• suggest values if the are known

Describe alternatives you've considered

Writing things manually

Additional context

Directives are contained in a single jar we can use https://index.scala-lang.org/virtuslab/scala-cli/artifacts/directives/1.5.1
and https://github.com/VirtusLab/scala-cli/tree/main/modules/directives/src/main/scala/scala/build/preprocessing/directives
We should asses how we can use that for completions

Search terms

using directives hover completions