Add synthetics and symbol information for semanticdb in Scala 3

Semantic information is essential for rich IDE features such as go-to-implementation and rename. Metals utilizes semantic information extracted from the compiler in a SemanticDB data model form to realize several features. However, SemanticDB generated from Scala3 lacks some kinds of information: Signature and Synthetic. Therefore some IDE features are not available in Scala3 projects.

The objective of this project is to improve the SemanticDB extractor for Scala3, and pave the way to better Scala3 support in Metals, scalafix, and other dev tools relying on SemanticDB.