Add Gradle Kotlin DSL

[lildude]

Description

This PR adds Gradle Kotlin DSL as a new language grouped with Gradle.

Closes #6469

Checklist:

• I am adding a new language.
  • The extension of the new language is used in hundreds of repositories on GitHub.com.
    • Search results for each extension:
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.gradle.kts
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • build.gradle.kts
    • Sample license(s): MIT
  • I have included a syntax highlighting grammar: [URL to grammar repo]
  • I have added a color
    • Hex value: #02303a
    • Rationale: It's the same colour as used for Gradle
  • I have updated the heuristics to distinguish my language from others using the same extension.

/cc mikewacker as author of #6469