Gets or sets a URI that points to a Markdown document to load and render.
Supports avares:// embedded resources, file://, and http/https schemes.
A #fragment scrolls to the matching anchor once the document has rendered; for
avares:// the fragment starts at the first #, so asset paths containing
# are not supported.
When both Source and Markdown are set,
Source takes precedence.
public Uri? Source { get; set; }