how to define library constant in gradle file

build.gradle(Module: app)

    //navigation
    implementation "androidx.navigation:navigation-fragment-ktx:$nav_version"
    implementation "androidx.navigation:navigation-ui-ktx:$nav_version"

    //room database

    implementation "androidx.room:room-ktx:$room_version"
    kapt "androidx.room:room-compiler:$room_version"
    implementation "androidx.lifecycle:lifecycle-extensions:$lifecycle_version"

build.gradle(project: )

//always keep buildscript above the plugin section. 
buildscript {
    ext {
        nav_version = "2.6.0"
        room_version = "2.5.2"
        lifecycle_version = "2.2.0"
    }
}