Create the XmlElement bindings

[LSViana]

Implement the bindings for the XmlElement struct based on the exposed bindings.

• [x] Create class definition
• [x] yxmlelem
• [x] yxmlelem_tag
• [x] yxmlelem_string
• [x] yxmlelem_insert_attr
• [x] yxmlelem_remove_attr
• [x] yxmlelem_get_attr
• [x] yxmlelem_attr_iter
• [x] yxmlattr_iter_destroy
• [x] yxmlattr_iter_next
• [x] yxmlelem_child_len
• [x] yxmlelem_insert_text
• [x] yxmlelem_insert_elem
• [x] yxmlelem_remove_range
• [x] yxmlelem_get
• [x] yxml_next_sibling
• [x] yxml_prev_sibling
• [x] yxmlelem_parent
• [x] yxmlelem_first_child
• [x] yxmlelem_tree_walker
• [x] yxmlelem_tree_walker_destroy
• [x] yxmlelem_tree_walker_next
• [x] yxmlelem_observe
• [x] yxmlelem_unobserve
• [x] yxmlelem_event_target
• [x] yxmlelem_event_path
• [x] yxmlelem_event_delta
• [x] yxmlelem_event_keys
• [x] yobserve_deep
• [x] yunobserve_deep

For each bound method, the implementation and unit tests must be written.

[LSViana]

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.

[LSViana]

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).