Create msbuild-test.yml github action

This tests out the github actions system, which is quite weird and undocumented. It says this only works with .NET core by default but also supports "4.5.1" as a version. It also says we aren't in the right github actions beta, so this shouldn't work at all.

.github/msbuild-test.yml

@@ -0,0 +1,13 @@
+name: MSBuild Test
+
+on: [push]
+
+jobs:
+  build:
+    runs-on: windows-latest
+    steps:
+    - uses: actions/checkout@master
+    - uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: '4.5.1'
+    - run: dotnet build --configuration Release ./TShockAPI/TShockAPI.csproj