[MC-221421] A list tag can be modified during insertion into itself

The bug

While all elements of a list tag are being inserted into all elements of the same list tag, the source list tag is modified.

How to reproduce

data modify storage mc-221421: list set value [[], []]

data modify storage mc-221421: list[] append from storage mc-221421: list[]

```
data get storage mc-221421: list
```

Expected behavior

Storage mc-221421: has the following contents: [[[], []], [[], []]]

The tags [], [] are inserted into each tag.

Actual behavior

Storage mc-221421: has the following contents: [[[], []], [[[], []], [[[], []]]]]

The tags [], [] are inserted into the first tag.
The modified first tag is inserted into the second tag.
The modified second tag is inserted into the second tag.

Code analysis

net.minecraft.server.commands.data.DataCommands.java

private static int insertAtIndex(int index, CompoundTag tag, NbtPath path, List<Tag> sources) throws CommandSyntaxException {
    var targets = path.getOrCreate(tag, ListTag::new);
    var result = 0;

    for (var target : targets) {
        if (!(target instanceof CollectionTag)) {
            throw ERROR_EXPECTED_LIST.create(target);
        }

        var success = false;
        var collection = (CollectionTag<?>) target;
        var cursor = index < 0 ? collection.size() + index + 1 : index;

        for (var source : sources) {
            try {
                if (collection.addTag(cursor, source.copy() /* Copying should be done in batches outside of the sources iteration. */)) {
                    ++cursor;
                    success = true;
                }
            } catch (IndexOutOfBoundsException e) {
                throw ERROR_INVALID_INDEX.create(cursor);
            }
        }

        result += success ? 1 : 0;
    }

    return result;
}