Improved support for textmate definition for kotlin dsl

[per-steinar]

I've made some improvements to the Kotlin grammar based on the official documentation and created a PR in the buildship repo.
It has been siting there for a month. Is there any interest in me porting the content of the PR?

See: eclipse/buildship#1273

[fbricon]

Sure, feel free to open a PR for https://github.com/redhat-developer/vscode-java/blob/master/language-support/kotlin/kotlin.tmLanguage.json