Add solidity support to languages.go

[douglance]

Summary

• Added Solidity language support for syntax highlighting in slides
• Added .sol file extension mapping with solc compiler 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)

Test plan

• Built and tested locally with go test ./internal/code/...
• Verified compilation with go build
• Test with actual Solidity code blocks in markdown presentations

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