fsxc: default to warnings as errors

Fixes #32

ReadMe.md

@@ -7,7 +7,7 @@ THIS REPO IS FACING A COMPLETE OVERHAUL/REVAMP/RENOVATION IN ORDER TO SUPPORT .N
 Unfinished tasks so far:
 * Revamp this ReadMe.md file to remove any mentions to Mono or the legacy .NET4.x framework.
 * Remove legacy framework support (so that build system can converge into .fsx files instead of autotools in Unix + fsx in Windows).
-* Make fsxc always enable warnAsError and fsx always disable it.
+* Allow fsxc && fsx disable warnAsError (via --w flag? or --ignore-warnings).
 * After doing the above, make both fsx and fsxc always enable warnAsError for the FS0020 warning described in https://stackoverflow.com/questions/38202685/fsx-script-ignoring-a-function-call-when-i-add-a-parameter-to-it
 * Try creating VMs for CI that uninstall .NETCore/.NET6 completely (not just the dotnet executable removal hack), to make sure legacy framework build still works there.
 * Try creating VMs for CI that uninstall Mono/.NET4.x completey (e.g. for macOS see: https://github.com/mono/website/commit/490797429d4b92584394292ff69fbdc0eb002948 )

fsxc/Fsxc.fs

@@ -710,6 +710,9 @@ let fsi = { CommandLineArgs = System.Environment.GetCommandLineArgs() }
     <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
   </PropertyGroup>
 
+  <PropertyGroup>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+  </PropertyGroup>
 """
 
             let initialProjectContents = initialProjectContents