Switch the core-api to be non-null by default

We'll do this everywhere eventually, but much easier to do it
incrementally:

 - Use checker framework to default all field/methods/parameters to
   @Nonnull.

 - Start using ErrorProne[1] and NullAway[2] to check for possible null
   pointer issues. I did look into using CheckerFramework, but it's much
   stricter (i.e. it's actually Correct). This is technically good, but
   is a much steeper migration path, which I'm not sure we're prepared
   for yet!

[1]: https://github.com/google/error-prone
[2]: https://github.com/uber/NullAway

buildSrc/build.gradle.kts

@@ -9,6 +9,7 @@ repositories {
 }
 
 dependencies {
+    implementation(libs.errorProne.plugin)
     implementation(libs.kotlin.plugin)
     implementation(libs.spotless)
 }