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.

NullHighlighter.h

@@ -0,0 +1,13 @@
+// NullHighlighter.h - default highlighter that emits a single Default span per line
+#pragma once
+
+#include "LanguageHighlighter.h"
+
+namespace kte {
+
+class NullHighlighter final : public LanguageHighlighter {
+public:
+    void HighlightLine(const Buffer &buf, int row, std::vector<HighlightSpan> &out) const override;
+};
+
+} // namespace kte