Add warnings

[panglesd]

Add warnings for many problems:

• Parsing problems in actions
• Unknown attributes (neither HTML standard, nor slipshow-specific)
• ID that link nowhere
• ID given multiple times
• ID that point to the wrong type of block (eg exec on something that is not a slip-script)
• Frontmatter parsing failures
• Missing files

Fix #81 #82 #84 #110 which completes #80.

This requires good locations, so I made a few fixes in the vendored cmarkit.

The reporting rendering is done using grace:

$ slipshow compile slip.md
warning[FSError]: file 'img.png' could not be read: img.png: No such file or directory
    ┌─ slip.md:3:25
  3 │  A missing file: ![image](img.png)
    │                          ^^^^^^^^^ 

warning[DupID]: ID id1 is assigned multiple times
    ┌─ slip.md:10:9
  7 │  Hello1{#id1}
    │          ^^^ 
  8 │  Hello2{#id1}
    │          ^^^ 
  9 │  
 10 │  Hello3{#id1}
    │          ^^^ 

It remains to make those warnings nicely visible in serve mode, and to add comprehensive tests.