Switch the core library to be non-null by default

See comments in c8c128d335 for further
details. This requires /relatively/ few changes - mostly cases we were
missing @Nullable annotations.

projects/core-api/build.gradle.kts

@@ -18,6 +18,7 @@ dependencies {
     compileOnly(project(":mc-stubs"))
     compileOnlyApi(libs.jsr305)
     compileOnlyApi(libs.checkerFramework)
+    compileOnlyApi(libs.jetbrainsAnnotations)
 
     "docApi"(project(":"))
 }

projects/core-api/src/main/java/dan200/computercraft/api/lua/IArguments.java

@@ -5,6 +5,8 @@
  */
 package dan200.computercraft.api.lua;
 
+import org.jetbrains.annotations.Contract;
+
 import javax.annotation.Nullable;
 import java.nio.ByteBuffer;
 import java.util.Map;
@@ -387,7 +389,9 @@ public interface IArguments {
      * @return The argument's value, or {@code def} if none was provided.
      * @throws LuaException If the value is not a string.
      */
-    default String optString(int index, String def) throws LuaException {
+    @Nullable
+    @Contract("_, !null -> !null")
+    default String optString(int index, @Nullable String def) throws LuaException {
         return optString(index).orElse(def);
     }
 
@@ -399,7 +403,9 @@ public interface IArguments {
      * @return The argument's value, or {@code def} if none was provided.
      * @throws LuaException If the value is not a table.
      */
-    default Map<?, ?> optTable(int index, Map<Object, Object> def) throws LuaException {
+    @Nullable
+    @Contract("_, !null -> !null")
+    default Map<?, ?> optTable(int index, @Nullable Map<Object, Object> def) throws LuaException {
         return optTable(index).orElse(def);
     }
 }

projects/core-api/src/main/java/dan200/computercraft/api/peripheral/IComputerAccess.java

@@ -86,6 +86,7 @@ public interface IComputerAccess {
      * @see #unmount(String)
      * @see IMount
      */
+    @Nullable
     String mountWritable(String desiredLocation, IWritableMount mount, String driveName);
 
     /**