switch to kotlin gradle dsl and make build gradle compliant

CHANGELOG.md

README.md

build.gradle

build.gradle.kts

gradle.properties

settings.gradle.kts

src/main/grammar/RelAstGrammar.bnf

...g/jastadd/tooling/RelAstGrammarCompletionContributor.java

...a/org/jastadd/tooling/RelAstGrammarFindUsageProvider.java

...main/java/org/jastadd/tooling/RelAstGrammarReference.java

src/main/java/org/jastadd/tooling/RelAstGrammarUtil.java

...g/jastadd/tooling/psi/RelAstGrammarTypeDeclExtension.java

...tadd/tooling/psi/RelAstGrammarTypeReferenceExtension.java

...rg/jastadd/tooling/psi/impl/RelAstGrammarPsiImplUtil.java

.../tooling/psi/impl/RelAstGrammarTypeDeclImplExtension.java

...ing/psi/impl/RelAstGrammarTypeReferenceImplExtension.java

src/main/resources/META-INF/plugin.xml