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/src/main/java/dan200/computercraft/core/util/Nullability.java

@@ -0,0 +1,26 @@
+/*
+ * This file is part of ComputerCraft - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2022. Do not distribute without permission.
+ * Send enquiries to dratcliffe@gmail.com
+ */
+package dan200.computercraft.core.util;
+
+import javax.annotation.Nullable;
+
+public final class Nullability {
+    private Nullability() {
+    }
+
+    /**
+     * An alternative to {@link java.util.Objects#requireNonNull(Object)}, which should be interpreted as an assertion
+     * ("this case should never happen") rather than an argument check.
+     *
+     * @param object The object to check, possibly {@literal null}.
+     * @param <T>    The type of the object to check
+     * @return The checked value.
+     */
+    public static <T> T assertNonNull(@Nullable T object) {
+        if (object == null) throw new NullPointerException("Impossible: Should never be null");
+        return object;
+    }
+}