Enable unity build to speed up compilation

[ranisalt]

Pull Request Prelude

• I have followed proper The Forgotten Server code styling.
• I have read and understood the contribution guidelines before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

Unity build will batch several files to be compiled at once (as if they were #included into one), which leads to faster builds by reducing the number of times header files are processed.

[MillhioreBT]

I don't know why the unit test failed, but this really makes it compile faster.

[Zbizu]

can be enabled in Windows also by adding this to .vcxproj
tested on most recent master branch (a904904) and it compiles

diff --git a/vc17/theforgottenserver.vcxproj b/vc17/theforgottenserver.vcxproj
index 0d040e59..4798b457 100644
--- a/vc17/theforgottenserver.vcxproj
+++ b/vc17/theforgottenserver.vcxproj
@@ -39,11 +39,13 @@
     <ConfigurationType>Application</ConfigurationType>
     <UseDebugLibraries>false</UseDebugLibraries>
     <PlatformToolset>v143</PlatformToolset>
+       <EnableUnitySupport>true</EnableUnitySupport>
   </PropertyGroup>
   <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|x64'" Label="Configuration">
     <ConfigurationType>Application</ConfigurationType>
     <UseDebugLibraries>false</UseDebugLibraries>
     <PlatformToolset>v143</PlatformToolset>
+       <EnableUnitySupport>true</EnableUnitySupport>
   </PropertyGroup>
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.props" />
   <ImportGroup Label="ExtensionSettings">