Create the XmlElement bindings about ydotnet HOT 2 CLOSED

The bindings are done but I'm not merging these changes to main yet because I need to check with the team why yxmlelem_parent is returning null if the reference to the XmlElement instance is retrieved from an Output cell.

from ydotnet.

Some follow-up issues were also fixed within this issue's scope:

• Defined UTF-16 as the default encoding when creating new documents (and updated affected unit tests);
• Fixed reading and writing strings on extern calls (they're now properly formatted as UTF-8 on both directions);
• Centralized some repeated calls to Marshal.SizeOf<T>();
• Also pushed some fixes to my fork of y-crdt.

There's still an open discussion with the team about how to create root-level XmlElement instances (deciding between yxmlelem() or yxmlfragment()) but the fixes, if needed, will be applied by a future PR (probably in the context of the XmlText bindings).

from ydotnet.