Markdown: Revert ProjectionBuffer hacks

This reverts commits 70730f404bf8543d34cd30c835e984be359a1032 and cf2cf3836402e586e3244ecd3d7aadcd2eeb4c10.

Conflicts:
	EditorExtensions/Classifications/Markdown/BufferGenerator.cs
	EditorExtensions/Classifications/Markdown/CodeLanguageEmbedders.cs
	EditorExtensions/Classifications/Markdown/MarkdownParser.cs
Markdown: Make parser return entire blocks

Allows us to locate code blocks without
per-line Artifacts, so HTML code blocks
can be handled by the outer buffer.

Doesn't work yet.
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.