feat: add nushell support

[niklas-heer]

I love this tool! I added a small modification to support my favourite shell, nushell, a new kind of shell where everything is structured data.

After some polishing and adding tests, I thought it was time to contribute my changes back.

Thank you so much for creating this great tool, @maaslalani! 🙏
I hope my contribution is up to your standards. Please let me know if I should change anything.

Summary

• Added nushell language support for syntax highlighting in slides
• Added nushell support for .nu file extension mapping with nu command
• Follows the same pattern as recent language additions (Haskell Add haskell support to languages.go #299, Scala add scala support using scala-cli #298)

Code Structure

slides/
├── internal/
│   ├── code/
│   │   ├── languages.go          # Enhanced with nushell support
│   │   ├── execute_test.go        # Added nushell execution tests
│   └── syntax/                   # New module for syntax highlighting
│       ├── nushell.go            # Nushell highlighting logic
│       └── nushell_test.go       # Comprehensive test suite
└── examples/
    └── code_blocks.md            # Enhanced nushell examples

🎨 How Syntax Highlighting Works

The Problem

• Chroma (used by Glamour) doesn't natively support nushell syntax highlighting
• Code blocks with nu identifiers showed as plain text

The Solution

1. Preprocessing: Before markdown is rendered, replace nu` with bash`
2. Fallback: Bash syntax highlighting provides reasonable highlighting for most nushell syntax
3. Future-Proof: System ready for native nushell support when available in Chroma

Code Flow

Markdown Input → Syntax Preprocessing → Glamour/Chroma → Highlighted Output
     ↓                    ↓                   ↓              ↓
```nu          →      ```bash        →    Chroma      →   Highlighted
echo "hi"              echo "hi"           Lexer           Code Block
```              ```              

Testing

• Built and tested locally with make test
• Verified compilation with make build
• Test with actual nushell code blocks in markdown presentations
• Added nushell syntax highlighting

make test
go test ./... -short
?   	github.com/maaslalani/slides	[no test files]
?   	github.com/maaslalani/slides/internal/cmd	[no test files]
?   	github.com/maaslalani/slides/internal/model	[no test files]
?   	github.com/maaslalani/slides/internal/server	[no test files]
ok  	github.com/maaslalani/slides/internal/code	0.752s
ok  	github.com/maaslalani/slides/internal/file	(cached)
ok  	github.com/maaslalani/slides/internal/meta	(cached)
ok  	github.com/maaslalani/slides/internal/navigation	(cached)
ok  	github.com/maaslalani/slides/internal/process	(cached)
ok  	github.com/maaslalani/slides/internal/syntax	0.552s
ok  	github.com/maaslalani/slides/styles	(cached)

🤖 Created with the help of Claude Sonnet 4