feature(nbt): CompoundBinaryTag#builder(existing)

[tal5]

Adds CompoundBinaryTag#createBuilder, which creates a builder from the compound tag's current contents.
This is useful if you're editing a couple of values at once, like compound.createBuilder().putInt("num", 6).putString("name", "Timothy").remove("old_value").build().

Might have merge conflicts with #1264, but I'll fix up whichever one is merged last.

[kashike]

Prefer CompoundBinaryTag.builder(existingTag)

[tal5]

Prefer CompoundBinaryTag.builder(existingTag)

Switched to that style, with a new CompoundBinaryTag#asMap method (returns an unmodifiable view of the tag's contents as a map) for a cleaner implementation without casting to the impl type or other approaches - that method is nice to have either way imo, for more convenient usage with any general map utility methods.