Add GitHub Actions configuration file to enable CI

There are several builds defined here, and a few more sketched out that
don't quite work.

Most of the steps call out to external shell or batch files,
to make things easier to keep track of.

.github/workflows/scripts/win/msvc-build.bat

@@ -0,0 +1,9 @@
+
+vcpkg integrate install
+
+setlocal enabledelayedexpansion
+
+for /f "usebackq tokens=*" %%i in (`vswhere -latest -requires Microsoft.Component.MSBuild -find MSBuild\**\Bin\MSBuild.exe`) do (
+	"%%i" -clp:ForceConsoleColor -target:Build -property:Configuration=Release -property:Platform=%1 "proj/vs2017/Blades of Exile.sln"
+	exit /b !errorlevel!
+)