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NetBeans 12.x and later already provide some basic support via in the java/kotlin.editor module. One can open a .kt file via the Windows→Favourites tab.This should be the plugin that should be further developed. It uses Kotlin.tmLanguage.json. This JavaDoc explains how to integrate a TextMate lexer with NetBeans.
With this plugin one can open a .kt file via the Windows→Favourites tab.
There are also contrib/groovy.grails and contrib/groovy.grailsproject.
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