Update natvis to include a few tweaks

[dmachaj]

Why is this change being made?

I set up the natvis in a new project that is using rapidyml and it was missing the primary type that we are using (ConstNodeRef). Additionally I discovered that the children synthetic visualization for NodeRef/ConstNodeRef was not being parsed correctly. The order of operations on the bitwise operator was causing the condition to always evaluate to false.

Briefly summarize what changed

• Duplicate the NodeRef visualization as ConstNodeRef. ConstNodeRef does not have an m_seed field so it cannot be exactly identical or else it fails to apply.
• Tweak NodeData::m_parent to special-case NONE instead of printing uint64(-1).
• Fix the [children] expansion in NodeRef/ConstNodeRef.
• Expand elm to element. (This one is arguable; I'm tempted to expand them all out so they are easier to understand without abbreviation).

I also tried to get NodeData to have a synthetic child with the expansion of the sub-elements (e.g. KEYMAP can print the keys in the map). Unfortunately this does not seem to be possible because the NodeData has the raw indexes but no pointer to the memory allocation.

How was this change tested?

I have a ~10 line program using rapidyml along with an OpenAPI.yml file from a public sample.

Closes #380

[biojppm]

LGTM. Thanks!