Markdown: Fix block-based artifacts

Changes parser to return regular Artifacts
for non-block code lines. (this removes the
MarkdownArtifact class)

Changes outliner to skip single-line indented
code blocks but not skip any fenced blocks.

Add more test cases.

All unit tests pass.
Not tested in the editor yet.
Markdown: Fix Outlining ProjectionBuffers

Edge inclusive projection source span
must cover entire source TextBuffer.