Add syntax highlighting infrastructure

- Introduced `HighlighterRegistry` with support for multiple language highlighters (e.g., JSON, Markdown, Python).
- Added `JsonHighlighter` implementation for basic JSON syntax highlighting.

LispHighlighter.h

@@ -0,0 +1,17 @@
+// LispHighlighter.h - simple Lisp/Scheme family highlighter
+#pragma once
+
+#include "LanguageHighlighter.h"
+#include <unordered_set>
+
+namespace kte {
+
+class LispHighlighter final : public LanguageHighlighter {
+public:
+    LispHighlighter();
+    void HighlightLine(const Buffer &buf, int row, std::vector<HighlightSpan> &out) const override;
+private:
+    std::unordered_set<std::string> kws_;
+};
+
+} // namespace kte