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

projects/core-api/build.gradle.kts

@@ -1,6 +1,7 @@
 plugins {
     id("cc-tweaked.java-convention")
     id("cc-tweaked.publishing")
+    id("cc-tweaked.errorprone")
     id("cc-tweaked")
 }