Explicitly disable implicit namespace imports (#1546)

This also reverts us back to language version 9.0.

This reverts commit e29979c.

Author: andyleejordan

PowerShellEditorServices.Common.props

@@ -4,15 +4,17 @@
     <VersionSuffix></VersionSuffix>
     <Company>Microsoft</Company>
     <Copyright>© Microsoft Corporation.</Copyright>
-    <LangVersion>10.0</LangVersion>
+    <LangVersion>9.0</LangVersion>
     <PackageTags>PowerShell;editor;development;language;debugging</PackageTags>
     <PackageLicenseUrl>https://raw.githubusercontent.com/PowerShell/PowerShellEditorServices/master/LICENSE</PackageLicenseUrl>
     <PackageRequireLicenseAcceptance>true</PackageRequireLicenseAcceptance>
     <RepositoryType>git</RepositoryType>
     <RepositoryUrl>https://github.com/PowerShell/PowerShellEditorServices</RepositoryUrl>
     <DebugType>portable</DebugType>
     <!-- See: https://docs.microsoft.com/en-us/dotnet/fundamentals/code-analysis/overview -->
-    <EnableNETAnalyzers>true</EnableNETAnalyzers>
     <!-- TODO: Enable `EnforceCodeStyleInBuild` -->
+    <EnableNETAnalyzers>true</EnableNETAnalyzers>
+    <!-- See: https://docs.microsoft.com/en-us/dotnet/core/compatibility/sdk/6.0/implicit-namespaces -->
+    <DisableImplicitNamespaceImports>true</DisableImplicitNamespaceImports>
   </PropertyGroup>
 </Project>