[Chapel Merge] Update highlight/README.md to provide a more compl

Branch: refs/heads/master
Revision: 035e96c
Author: bradcray
Log Message:

Merge pull request #17361 from bradcray/highlight-doc-adjustments

Update highlight/README.md to provide a more complete list of highlighters

[reviewed by @ronawho]

This makes the top-level highlight/README.md file a bit more precise about what Chapel highlighters are supported. While here, I also cleaned up some things about the geany README.md: