Split declarations and definitions to improve compile times when editing (#13332)

* Split FileStream declarations and definitions

* Split JobPool declarations and definitions

* Split StringBuilder declarations and definitions

* Split StringReader declarations and definitions

* Split ZoomLevel declarations and definitions

* Fix missing include in FileClassifier.cpp

* Remove pragma once from source files

* Fix missing include in StringBuilder.h

* Update Xcode project

* Fix compilation of tests

Co-authored-by: Michael Steenbeek <m.o.steenbeek@gmail.com>

test/tests/LanguagePackTest.cpp

@@ -15,6 +15,9 @@
 #include <gtest/gtest.h>
 
 #ifndef _WIN32
+#    include "openrct2/core/File.h"
+#    include "openrct2/platform/platform.h"
+
 const language_descriptor LanguagesDescriptors[] = {};
 #endif
 
@@ -124,3 +127,20 @@ const unsigned char LanguagePackTest::LanguageZhTW[] = {
     0x59, 0x20, 0x20, 0x20, 0x20, 0x3a, 0xe6, 0xaf, 0x8f, 0xe8, 0xbb, 0x8a, 0xe5, 0x8d, 0xa1, 0x34, 0xe4, 0xbd, 0x8d, 0xe4,
     0xb9, 0x98, 0xe5, 0xae, 0xa2, 0x0a, 0x00,
 };
+
+#ifndef _WIN32
+bool platform_ensure_directory_exists(const utf8* path)
+{
+    return true;
+}
+
+bool platform_directory_exists(const utf8* path)
+{
+    return true;
+}
+
+bool File::Exists(const std::string& path)
+{
+    return true;
+}
+#endif