release: Rush v1.2.0 - Githubissues

Changelog

feat: First-class Kotlin language support
feat: Java 8 language features support (lambda expressions, method references, etc.)
feat: rush upgrade command
feat: Markdown support in the extension description
feat: Auto-completion for the metadata file (rush.yml) in select IDEs
fix: Having no block annotation in the source file would crash Rush
fix: Multiline extension description won't show as expected in the builder
fix: Warnings would get mixed up with errors
fix: Processing extension would sometimes fail on macOS and Linux
fix: Sometimes errors would have the phrase src in front of them
fix: Optimized extensions would sometimes clash with each other
fix: Migrated extensions without an icon image won't build
misc: Slightly changed terminal-UI
misc: Improved validation for the metadata file (rush.yml)
misc: Deprecated rush-init in the favor of install scripts
misc: Deprecated release and license_url fields