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.

MarkdownHighlighter.h

@@ -0,0 +1,14 @@
+// MarkdownHighlighter.h - simple Markdown highlighter
+#pragma once
+
+#include "LanguageHighlighter.h"
+
+namespace kte {
+
+class MarkdownHighlighter final : public StatefulHighlighter {
+public:
+    void HighlightLine(const Buffer &buf, int row, std::vector<HighlightSpan> &out) const override;
+    LineState HighlightLineStateful(const Buffer &buf, int row, const LineState &prev, std::vector<HighlightSpan> &out) const override;
+};
+
+} // namespace kte