Set the container type in LanguagePack::FromFile to std::string

The code interpreting fileData assumes the string has a valid null terminator,
which is not guaranteed by std::vector, but is guaranteed by std::string.

src/openrct2/localisation/LanguagePack.cpp

@@ -70,7 +70,7 @@ public:
         Guard::ArgumentNotNull(path);
 
         // Load file directly into memory
-        std::vector<utf8> fileData;
+        u8string fileData;
 
         try
         {