Add textmate definition for kotlin dsl

[jdneo]

fix #3334

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After:

[fwcd]

Hm, while it's nice to have *.gradle.kts support in the upstream Java extension, it potentially also overrides the grammar of other Kotlin extensions (disclaimer: I'm the maintainer of this Kotlin extension). This is a bit frustrating since bugs in the grammar can potentially no longer be addressed by the Kotlin extensions themselves.

Perhaps, as an intermediate fix, the language support for Kotlin via this extension could be scoped to .gradle.kts files, so it doesn't affect other Kotlin files (which aren't handled by this language server anyway)?

Edit: Ah, I think the extension already does this, but uses source.kotlin as a scope. Ideally, a different scope should be used in that case, since those grammar apparently may still clash if another extension uses source.kotlin (?).