Make WASM packages stay preview releases in 3.1. (dotnet/extensions#2503)

Fixes https://github.com/aspnet/AspNetCore/issues/14623\n\nCommit migrated from dotnet/extensions@b195e40

Author: pranavkm

src/JSInterop/Mono.WebAssembly.Interop/Directory.Build.props

@@ -0,0 +1,9 @@
+<Project>
+  <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory)..\, Directory.Build.props))\Directory.Build.props" />
+
+  <PropertyGroup>
+    <!-- Override prerelease label even in the final build -->
+    <PreReleaseVersionLabel>$(BlazorWasmPreReleaseVersionLabel)</PreReleaseVersionLabel>
+    <DotNetFinalVersionKind />
+  </PropertyGroup>
+</Project>