Add Maven Build Configuration

[prmr]

Add support for building the project with Maven as an alternative to default building with Eclipse.

• The project should remain compilable and runnable in Eclipse with no additional configuration.
• The build configuration (pom.xml) should respect the current directory structure.
• The build configuration should include options for packaging JetUML as a thin or fat jar.
• Migrate to Java 25.
• Update the developer docs in consequence