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.cc

@@ -0,0 +1,16 @@
+#include "NullHighlighter.h"
+#include "Buffer.h"
+
+namespace kte {
+
+void NullHighlighter::HighlightLine(const Buffer &buf, int row, std::vector<HighlightSpan> &out) const
+{
+    const auto &rows = buf.Rows();
+    if (row < 0 || static_cast<std::size_t>(row) >= rows.size()) return;
+    std::string s = static_cast<std::string>(rows[static_cast<std::size_t>(row)]);
+    int n = static_cast<int>(s.size());
+    if (n <= 0) return;
+    out.push_back({0, n, TokenKind::Default});
+}
+
+} // namespace kte