Add syntax highlight for Nunjucks templating

Description

Nunjucks is a rich and powerful templating language for JavaScript. It uses for backend / frontend development.

Proposal

It would be great to support of syntax highlight for Nunjucks.
In particular it means highlighting .njk files using Twig preset.

Links / references

• https://mozilla.github.io/nunjucks/
• https://mozilla.github.io/nunjucks/templating.html#file-extensions
• https://twig.symfony.com/