Nullability annotations: Use JSpecify

[ljacqu]

Change from JetBrains annotations for @NotNull and @Nullable to JSpecify. Semantics of the annotations are mostly the same.

See if we can remove some @NotNull by implicitly defining it when no annotation is present...

[ljacqu]

This is interesting: https://github.com/google/guava/blob/master/guava/src/com/google/common/base/ElementTypesAreNonnullByDefault.java

[ljacqu]

#450 Jetbrains annotations in 26.0.0:

Added new experimental annotation: @NotNullByDefault

[ljacqu]

I think JSpecify is becoming the way to go. Keep Jetbrains for 2.0 but change to JSpecify in 2.1. Not sure how we can reduce the number of annotations without having a Java module but at the worse it will stay the same as now :)

Renamed and rewrote issue description to be about migrating to JSpecify.