lua,rawblock - recognize and process pandoc-reader:LANG blocks

[cscheid]

Add support for this syntax:

```{=pandoc-reader-html}
<strong>bold</strong>
```

A filter will catch those raw blocks and convert them via pandoc.read to Pandoc AST.

[posit-snyk-bot]

🎉 Snyk checks have passed. No issues have been found so far.

✅ security/snyk check is complete. No issues have been found. (View Details)

✅ license/snyk check is complete. No issues have been found. (View Details)