Allow the //> using exclude directive in an .sc script rather than just project.scala (but still only one place)

[Gedochao]

Fixes #3388

So with this we allow //> using exclude directives to live in either project.scala or any .sc script - but only in one spot at a time.

Checklist

• tested the solution locally and it works
• ran the code formatter (scala-cli fmt .)
• ran scalafix (./mill -i __.fix)
• ran reference docs auto-generation (./mill -i 'generate-reference-doc[]'.run)

How much have your relied on LLM-based tools in this contribution?

extensively

How was the solution tested?

included automated tests