Updated buildscript

[X1nto]

Here are the changes so far:

• Migrated build logic to Kotlin DSL
• Updated Kotlin to 1.5.0
• Updated AGP to 4.2.1
• Updated Gradle to 7.0.2
• Moved ThemerConstants.gradle content to buildSrc/ThemerConstants.kt
• Moved kotlin_version ext to buildSrc/Constants.kt
• Bumped up Compile SDK to 30
• Bumped up AndroidX AppCompat to 1.2.0
• Added android.useAndroidX = true and android.enableJetifier=true to gradle.properties
• Replaced JCenter repo with Maven Central since JCenter is now deprecated