Fix highlight package about flutter-code-editor

[alexeyinkin]

We are using this package for highlighting: https://pub.dev/packages/highlight
It is not maintained, and the authors do not respond to issues.

It is a port of a well-maintained JavaScript package: https://highlightjs.org

Maybe we should investigate the automation of porting this from JavaScript and make our own highlighting package.

[alexeyinkin]

The impact is that we rely on to-be-hidden service comments for special features like read-only blocks, initially collapsed blocks, and hidden blocks. And we rely on this highlighting package to parse the comments. If some code breaks highlighting, we cannot parse comments, and those features break.

A workaround is made in #28. If highlighting breaks, we fall back to a naive parsing that just searches for // or # depending on the language and considers everything from that sequence to the end of the string a comment. It gives false positives for

string s = "// not a comment";

and so it is unreliable.