1.0.0-rc6

• Compatible with GraalVM 19.3.0 (based on OpenJDK 8)
• Add Polyglot API Reference
• Introduce Interop class in GraalSqueak image (0991b90 and e340cab)
• Use default launchers provided by GraalVM
• Use mx to generate platform-specific GraalVM components (thanks to @michelou for the initial scripts)
• Various bugfixes and improvements (thanks to @fmateoc)
• Revise CI setup

All changes: 1.0.0-rc5...1.0.0-rc6