Support InlineSpan as customRender

[erickok]

Same as #298 but only the minimal required changes to make this work.

          customRender: {
            "bird": (RenderContext context, Widget child, attributes, _) {
              return TextSpan(text: "🐦");
            },
          },

Side note: I noticed that custom renders only work for tags with a proper, separate closing tag (not self-closing) but I guess that is in line with the html spec (and an implementation of the html parser in the end).

[ryan-berger]

This looks good to me. Definitely gives us a bit more control over how we render with a customRender

[ryan-berger]

@erickok Once conflicts are fixed I can merge this

[erickok]

Resolved.