Add d2-vim to Language Specific section

[alixander]

D2 is a modern diagram scripting language that turns text to diagrams. This plugin provides comprehensive vim support including syntax highlighting, formatting, validation, and playground integration.

Repository: https://github.com/terrastruct/d2-vim
D2 Language: https://d2lang.com

[alixander]

Hi @akrawchyk friendly ping, let me know if there's anything else needed