Parse Mill build.mill / build.sc

Mill is the second most common Scala build tool after sbt. Its build definition (`build.mill`, older projects use `build.sc`) declares dependencies as interpolated strings:

```
ivy"org.typelevel::cats-core:2.9.0"
ivy"org.slf4j:slf4j-api:1.7.36"
mvn"com.google.guava:guava:33.0.0-jre"
```

`::` is the Scala cross-version marker (equivalent to sbt `%%`), single `:` is a plain Maven coordinate. Same regex-parser approach as `build.sbt`.

The change is in `ecosyste-ms/bibliothecary` (`lib/bibliothecary/parsers/maven.rb`); filing here since that fork has issues disabled.

Start in ecosyste-ms/bibliothecary/lib/bibliothecary/parsers/maven.rb and compare the existing build.sbt parsing approach. Check how the parser handles interpolated dependency strings, then verify that build.mill and build.sc examples with both :: and single : coordinates are recognized correctly.

Written by the indexing model from the issue text.