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/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