Add language: dyff

## Language name
dyff

## URL of example repository
https://github.com/homeport/dyff

## URL of syntax highlighting grammar
TBD

## Most popular extensions
unaware of extensions, if any

## Detected language
N/A(?)

I would love to have dyff highlighting in code blocks on PRs. [Initially I went down the path](https://github.com/adam-p/markdown-here/wiki/Markdown-Cheatsheet#code-and-syntax-highlighting) of having a [highlight.js addition](https://github.com/homeport/dyff/issues/289) for this language, but then [the github docs seem to suggest liguist as the way to go](https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-and-highlighting-code-blocks#syntax-highlighting) - to which I am opening this issue here.

Start by reviewing the dyff example repository, the linked dyff issue 289, and the syntax-highlighting guidance in the issue. Determine the grammar, extensions, and detected language needed for Linguist; done means dyff code blocks receive syntax highlighting in GitHub pull requests.

Written by the indexing model from the issue text.