Initial project update.

- Updated github actions workflow to build using .NET Core on Linux, Windows and Mac.
- Updated Raylib-cs.sln with project name changes.
- Added experimental netfx.props to help with building using Mono on Linux.
- Added app.config.

.github/workflows/dotnetcore.yml

@@ -4,8 +4,11 @@ on: [push, pull_request]
 
 jobs:
   build:
-
-    runs-on: ubuntu-latest
+    name: Build on dotnet using ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest, macOS-latest]
 
     steps:
       - uses: actions/checkout@v1
@@ -14,4 +17,4 @@ jobs:
         with:
           dotnet-version: 3.1.100
       - name: Build with dotnet
-        run: dotnet build --configuration Release
+        run: dotnet build --framework=netcoreapp3.1 -c Release