report unused Import and have them dimmed

[RodgeFu]

Clear and concise description of the problem

There should be warning for unused Import and have them dimmed in IDE

Checklist

• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[RodgeFu]

we can't 100% determine whether a "import" is needed based on type reference because lib and emitters can potentially modify type graph in either OnValidate or OnEmit, so without these information, removing "import" may impact the compilation unexpectedly.
Further investigation is needed. Potential solution:

• Instead of the default way showing unused import, we need find a different way to show these info to end user more accurately so that user can make better judgement (more design needed)
• Leverage the "type-only" lib feature from compiler when it's ready so that we can 100% sure an import is needed or now.

set pri to P1.5 and we will look into this and double confirm the "type-only" lib feature status after all P1 is done.