add scdoc syntax highlighting

[mossheim]

This PR adds highlighting for schelp files. The choices of highlighting links are totally arbitrary and open to discussion. I tried to follow the markup syntax as best I could; features include:

• highlighting for all scdoc tags
• specialized formatting for strong, emphasis, soft, link, anchor, image regions
• you get highlighting of code blocks "for free" using syn include
• nested trees, tables, other structures
• notes and warnings containing nested modal tags (strong::, etc) are highlighted

[mossheim]

The only thing I don't like that I can't figure out how to fix is highlighting within inline code sections. Right now in code::test::, the sc highlighting picks up test: as a keyword selector, presumably because it's using lookahead in its regex(?). I tried several times to fix this but none of them worked. I'm considering just turning it off for inline code and leaving it only for code blocks.

[dvzrv]

This looks very useful!
@brianlheim if you disable this for code::test::, can you add a a comment saying why (e.g. test: gets picked up as keyword selector - needs further investigation)?

[capocasa]

Great stuff Brian! Mentioned issue does not seem to be a blocker to me.