Add Display method for Node variant *name*

[noahprice-dev]

I am working with this crate to perform MD to SVG translation. Id like to be able to create warnings for unsupported/Not Yet Supported MD types that provide the markdown type in an error message, however this currently requires a lengthy match on each unsupported type in my own crate or interpolating the Debug display.

Id love to see something like strum implemented for the Node enum or a simple kind method on the Node enum to communicate what type it is. This shouldn't replace the existing infrastructure for matching, it's just to get a cleaner identity for my error messages.

Something like

pub rn node_kind(&self) -> String {
  match self {
    Self::Root(_) => String::new("Root"),
    Self::Paragraph(_) => String::new("Paragraph")
    //.. etc

I think that if including strum is deemed to heavy even as a feature flag, that a simple display impl could be used with the string name of the variant. Il take a crack at this in a PR, but wanted to put it out if others have a similar use case or desire this functionality.

Cheers!