gradle.lang: recognize all *.gradle files

Some examples of *.gradle files:

• https://docs.gradle.org/6.0.1/userguide/plugins.html#sec:script_plugins
• https://github.com/gradle/gradle/blob/v6.0.1/gradle/versioning.gradle
• https://github.com/JetBrains/kotlin-native/blob/v1.3.60/gradle/kotlinGradlePlugin.gradle