Restore ghc-syntax-highlighter & revdeps

[DanBurton]

Checklist:

• Meaningful commit message, eg add my-cool-package (please not mention build-constraints.yml)

• At least 30 minutes have passed since uploading to Hackage

• On your own machine, in a new directory, you have successfully run the following set of commands (replace $package with the name of the package that is submitted, and $version with the version of the package you want to get into Stackage):

  stack unpack $package-$version  # $version is optional
  stack init --resolver nightly
  stack build --resolver nightly --haddock --test --bench --no-run-benchmarks
  

(Tested ghc-syntax-highlighter only; haddocks untested)

[juhp]

Merged, thanks