Harper is a grammar checker. (GitHb repo)
Harper's curated dictionary is very simple.
The first line is a number specifying the approximate number of entries (lines) in the dictionary.
Each line after that starts with a mandatory word which is optionally followed by a series of annotation flags, separated from the word by a /.
This syntax highlighter also supports optional comments which are separated from the word and annotation by whitespace.
To learn about the annotation flags please consult the official documentation.
The author knows nothing about VS Code syntax highlighter extensions and nothing about TextMate. So please feel free to contribute improvements that turn this into a proper VS Code syntax highlighter.