Add syntax highlighting and high-contrast themes

[simwai]

Add real‑time syntax highlighting for 17 languages while preserving existing terminal‑compatible rendering and performance. Colours meet enhanced contrast goals of ≥ 7.7:1 for comments and ≥ 9:1 for all other tokens.

• Add $script:syntaxRules with pre-compiled regex for all supported languages.
• Extend theme system with 10 syntax foreground color keys.
• Add new high-contrast 'latte-contrast' light theme.
• Update Get-Language for broader extension and filename support.
• Implement Get-LineTokens and a per-line TokenCache for performance.
• Modify Build-EditorRow to apply syntax highlighting under selection.
• All existing tests pass.

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PR created automatically by Jules for task 13001582345927600502 started by @simwai

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