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/install-deps.bat

@@ -0,0 +1,2 @@
+
+vcpkg install zlib:%1-windows sfml:%1-windows opengl:%1-windows boost-any:%1-windows boost-dynamic-bitset:%1-windows boost-ptr-container:%1-windows boost-core:%1-windows boost-filesystem:%1-windows boost-system:%1-windows boost-date-time:%1-windows boost-chrono:%1-windows boost-math:%1-windows

.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!
+)

.github/workflows/scripts/win/run-tests.bat

@@ -0,0 +1,6 @@
+
+cd test
+
+@rem These are the same command-line arguments that are passed in the Xcode scheme.
+@rem If the scheme is changed, these should be updated too.
+"..\proj\vs2017\x64\Release\Tests.exe" -i --order lex

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

@@ -0,0 +1,8 @@
+
+setlocal enabledelayedexpansion
+
+for /f "usebackq tokens=*" %%i in (`vswhere -latest -products * -requires Microsoft.VisualStudio.Component.VC.Tools.x86.x64 -find **/Auxiliary/Build/vcvarsall.bat`) do (
+  %%i
+)
+
+scons