feat(d2): add D2 language support (SVG/PNG/TXT)

[AllenWu233]

What's added

• Add support to compiler.nvim
• Support 4 compilation options:
  1. Compile D2 to SVG
  2. Compile D2 to PNG
  3. Compile D2 to TXT (only file)
  4. Compile D2 to TXT (file + terminal output)
• Output files are generated in the same directory as the source .d2 file
• Follow the same code structure as c.lua (official template)

Tested

• D2 CLI version: v0.7.1
• Neovim version: v0.11.5
• OS: Arch Linux (6.17.9-zen1-1-zen)