Refactor syntax highlighting infrastructure and related classes.

- Moved all language highlighter implementations (`CppHighlighter`, `GoHighlighter`, `JsonHighlighter`, etc.), the engine, and registry to `syntax/`.

syntax/ForthHighlighter.h

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