Switch to JSpecify annotations

Now, hear me out, what if instead of having three @Nullable annotations,
we had *four*?

I've been wanting to switch away from javax.annoations for a while. The
library has been deprecated for ever and, unlike other @Nullable
annotations, the annotation is attached to the parameter/function
itself, rather than the type.

We use JSpecify rather than one of the alternatives (JetBrains,
CheckerFramework) mostly because it's what NullAway recommends. We keep
CheckerFramework around for @DefaultQualifier, and JB's for @Contract.

There are some ugly changes here — for instance, `@Nullable byte[]` is
replace by `byte @Nullable`, and `@Nullable ILuaMachine.Factory` is
`ILuaMachine.@Nullable Factory`. Ughr, I understand why, but it does not
spark joy :).

buildSrc/src/main/kotlin/cc-tweaked.java-convention.gradle.kts

@@ -84,7 +84,6 @@ sourceSets.all {
 
         options.errorprone {
             check("InvalidBlockTag", CheckSeverity.OFF) // Broken by @cc.xyz
-            check("InvalidParam", CheckSeverity.OFF) // Broken by records.
             check("InlineMeSuggester", CheckSeverity.OFF) // Minecraft uses @Deprecated liberally
             // Too many false positives right now. Maybe we need an indirection for it later on.
             check("ReferenceEquality", CheckSeverity.OFF)