Syntax highlighting in html view

[mariusc23]

Hi again! It would be great if syntax highlighting could be added to the html view. Perhaps matching graphiql coloring. Some ideas:

• descriptions could be gray
• properties blue
• values orange

[fabsrc]

Just saw that there is a PR which adds GraphQL syntax highlighting to highlight.js (highlightjs/highlight.js#3448).
After that PR is merged and released I can update graphql-schema-diff to enable GraphQL syntax highlighting.