Fixed JSON output for non-ASCII filenames

When exporting to JSON, the "soundFile" string wasn't correctly encoded if it contained non-ASCII characters.
In this case, the resulting JSON was invalid and couldn't be parsed.

src/exporters/JsonExporter.cpp

@@ -1,11 +1,17 @@
 #include "JsonExporter.h"
 #include "exporterTools.h"
+#include <utf8.h>
 
 using std::string;
 
 string escapeJsonString(const string& s) {
+	// JavaScript uses UTF-16 internally. As a result, character escaping in JSON strings is UTF-16-based.
+	// Convert string to UTF-16
+	std::u16string utf16String;
+	utf8::utf8to16(s.begin(), s.end(), std::back_inserter(utf16String));
+
 	string result;
-	for (char c : s) {
+	for (char16_t c : utf16String) {
 		switch (c) {
 		case '"':  result += "\\\""; break;
 		case '\\': result += "\\\\"; break;
@@ -15,10 +21,13 @@ string escapeJsonString(const string& s) {
 		case '\r': result += "\\r"; break;
 		case '\t': result += "\\t"; break;
 		default:
-			if (c <= '\x1f') {
+			{
+				bool needsEscaping = c < '\x20' || c >= 0x80;
+				if (needsEscaping) {
 					result += fmt::format("\\u{0:04x}", c);
 				} else {
-				result += c;
+					result += static_cast<char>(c);
+				}
 			}
 		}
 	}