add support for `json` filter

[jkrenge]

Adds support for json filter:

{{ item | json }}

---

• [x] I've run the tests with vendor/bin/phpunit
• [x] None of the tests were found failing
• [ ] I've seen the coverage report at build/coverage/index.html
• [x] Not a single line left uncovered by tests
• [x] Any coding standards issues were fixed with vendor/bin/php-cs-fixer fix

[jkrenge]

Apologies for my ignorance, I cannot assign reviewers — anything else I need to do for this PR? :)

[jkrenge]

@sanmai

LGTM and sorry for the holdback

No problem, thank you so much!