Phase 0: strip JetBrains template, register Jai language and file type

- delete demo tool window + message bundle, rewrite plugin.xml metadata
- add JaiLanguage, JaiFileType, file icon, JaiTokenTypes (from the compiler's
  own lexer: modules/Jai_Lexer/module.jai)
- add ./jaitest: runs tests and reports the JUnit XML, not just the exit code
- ignore .kotlin

.gitignore

@@ -1,4 +1,5 @@
 .gradle
 .idea
 .intellijPlatform
-build
+.kotlin
+build