Add lexer for Dhall language (@dhall-lang/dhall-lang)

[malik-n]

Is there an existing issue for this?

• [x] I have searched the existing issues

What is the missing lexer?

Dhall (https://github.com/dhall-lang/dhall-lang) is a programmable configuration language with a standardized syntax. Chroma currently does not have a dedicated lexer for Dhall. Resolving this issue would provide dhall support inside Gitea?

Links to existing syntax definitions

• Dhall Sublime Text: https://github.com/kukimik/dhall-sublime-syntax-highlighting
• Dhall official language repository: https://github.com/dhall-lang/dhall-lang