Migrate to Gradle

[freya022]

While Gradle is somehow slower (why does it make JARs that nothing uses) for the usual compile-run-test loop than Maven, at least when using IntelliJ, it should be better for the library due to:

1. The ability to hide implementation dependencies
2. Easier logic sharing across modules
3. Having advanced build logic in the same project, such as KSP processors
4. Configuring Dokka is not as fleshed out in Maven than it is in Gradle, especially to configure multi-module docs and versioning

This will go live alongside the JDA 6 update.

Breaking changes

• Moved Spring support to a separate module
  • Add the BotCommands-spring dependency, that's it
• Implementation dependencies are now exposed as runtime dependencies (previously were compile dependencies)
  • Reduces the classpath size for you, improving IDE and build performance, and avoiding you from (accidentally) depending on internal transitive dependencies

Changes

• Some Kotlin internals have been made public to facilitate modules, but as a reminder, anything in an internal package may be changed at any time