Refactor string conversion between RCT2 and UTF8

Use Win32 API for conversion on Windows for non-1252 code pages instead of built-in tables.

src/openrct2/core/String.cpp

@@ -18,6 +18,12 @@
 #include <stdexcept>
 #include <vector>
 
+#ifdef _WIN32
+#define NOMINMAX
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+#endif
+
 #include "../localisation/Language.h"
 #include "../util/Util.h"
 
@@ -517,4 +523,32 @@ namespace String
         size_t stringLength = endSubstr - startSubstr + 1;
         return std::string(startSubstr, stringLength);
     }
+
+    std::string Convert(const std::string_view& src, sint32 srcCodePage, sint32 dstCodePage)
+    {
+#ifdef _WIN32
+        // Convert from source code page to UTF-16
+        std::wstring u16;
+        {
+            int srcLen = (int)src.size();
+            int sizeReq = MultiByteToWideChar(srcCodePage, 0, src.data(), srcLen, nullptr, 0);
+            u16 = std::wstring(sizeReq, 0);
+            MultiByteToWideChar(srcCodePage, 0, src.data(), srcLen, u16.data(), sizeReq);
+        }
+
+        // Convert from UTF-16 to destination code page
+        std::string dst;
+        {
+            int srcLen = (int)u16.size();
+            int sizeReq = WideCharToMultiByte(dstCodePage, 0, u16.data(), srcLen, nullptr, 0, nullptr, nullptr);
+            dst = std::string(sizeReq, 0);
+            WideCharToMultiByte(dstCodePage, 0, u16.data(), srcLen, dst.data(), sizeReq, nullptr, nullptr);
+        }
+
+        return dst;
+#else
+        STUB();
+        return std::string(src);
+#endif
+    }
 }