[Highlighting] Allow custom colors for package #SCL-25408

[Siupahc]

What does this PR do?

This PR introduces customizable syntax highlighting for package references in the Scala plugin. Previously, package names in import statements and fully qualified references lacked a dedicated semantic color setting.

Key Changes

• Text Attribute Key: Added a new PACKAGE highlight info type and text attributes key, with a graceful fallback to the default identifier color to support all themes.
• Semantic Analysis: Updated ScalaColorsSchemeUtils to detect resolved PsiPackage elements and assign them the new PACKAGE highlight type.
• Color Settings UI: Exposed the new "Package" attribute in Settings > Editor > Color Scheme > Scala. The demo text to demonstrates package highlighting in action.
• Testing: Added testPackageHighlighting to ScalaColorSchemeAnnotatorTest to verify that package sequences are correctly identified and colored during semantic analysis.

Related YouTrack Issue

Fixes #SCL-25408