Use c++ in runtime src file

It makes string manipulation easier

src/runtime/document.cpp

@@ -0,0 +1,37 @@
+#include "tree_sitter/runtime.h"
+
+struct TSDocument {
+    TSParseFn *parse_fn;
+    const char **symbol_names;
+    const char *text;
+    TSParseError error;
+    TSTree *tree;
+};
+
+TSDocument * TSDocumentMake() {
+    return new TSDocument();
+}
+
+void TSDocumentSetUp(TSDocument *document, TSParseConfig config) {
+    document->parse_fn = config.parse_fn;
+    document->symbol_names = config.symbol_names;
+}
+
+void TSDocumentSetText(TSDocument *document, const char *text) {
+    TSParseResult result = document->parse_fn(text);
+    document->text = text;
+    document->tree = result.tree;
+    document->error = result.error;
+}
+
+TSTree * TSDocumentTree(const TSDocument *document) {
+    return document->tree;
+}
+
+const char * TSDocumentToString(const TSDocument *document) {
+    if (document->error.expected_inputs != NULL) {
+        return TSParseErrorToString(&document->error, document->text, document->symbol_names);
+    } else {
+        return TSTreeToString(document->tree, document->symbol_names);
+    }
+}