Fix: Convert language names to lowercase before Shiki highlighting

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This PR fixes the issue where Shiki syntax highlighting failed when language names were provided in uppercase format (e.g., "JSON", "Python") instead of the expected lowercase format.

Problem

Previously, passing uppercase language names to the highlighting function would result in errors like:

Language "JSON" is not supported by Shiki.
Language "Python" is not supported by Shiki.

This occurred because Shiki's bundledLanguages object contains language keys in lowercase (e.g., "json", "python"), but the code was performing case-sensitive comparisons without normalization.

Solution

Updated the getHighlighter and highlight functions in src/lib/highlight.ts to normalize language names to lowercase before processing:

1. Language validation: Convert input to lowercase before checking if it exists in the supported languages list
2. Cache consistency: Use normalized language names in cache keys for better efficiency
3. Highlighting: Pass normalized language names to Shiki's codeToHtml method

Benefits

• ✅ Supports both uppercase and lowercase language names seamlessly
• ✅ Maintains existing caching mechanism while improving cache efficiency
• ✅ Minimal code changes with no breaking changes
• ✅ Fallback behavior for unsupported languages remains unchanged

Example

// Both of these now work correctly:
await highlight('JSON', '{"test": true}');     // ✅ Works
await highlight('json', '{"test": true}');     // ✅ Already worked

await highlight('Python', 'print("hello")');  // ✅ Works  
await highlight('python', 'print("hello")');  // ✅ Already worked

Fixes #65.

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