Use the new signing features for RealSigning (#10)

Context: xamarin/yaml-templates#73

> [xamarin/yaml-templates#73] changes things in that nothing is signed
> and the nuget-signed artifact no longer gets created. In order to
> create that artifact, at least one of he 4 "control" parameters needs
> to be set.  If you have received approval to RealSign, then you can
> add the  `realSign` parameter as well.

The effect is two changes of consequence vs. bce1b7f & 3fa0c2e: 

 1. When NuGet packages are signed, a "real" signing certificate will
    be used, not a "test" signing certificate.  (This was the case
    with Mono.Terminal v5.4.1.)

 2. Builds for all tags and master commits are *no longer* signed.
    Instead, signing is an explicit AzDO operation after the build
    is complete.  This ensures manual review & approval of all
    signed artifacts.

.ci/azure-pipelines.yml

@@ -38,3 +38,6 @@ jobs:
     - template: sign-artifacts/jobs/v1.yml@internal-templates
       parameters:
         dependsOn: [ 'build' ]
+        realSign: true
+        signTags: true
+        signMain: true

LineEditor/LineEditor.csproj

@@ -3,14 +3,14 @@
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
     <PackageId>Mono.Terminal</PackageId>
-    <PackageVersion>5.4.1</PackageVersion>
+    <PackageVersion>5.4.2</PackageVersion>
     <Authors>Microsoft</Authors>
     <Copyright>© Microsoft Corporation. All rights reserved.</Copyright>
     <PackageIconUrl>https://github.com/mono/LineEditor/raw/master/icon_128x128.png</PackageIconUrl>
     <PackageLicenseUrl>https://github.com/mono/LineEditor/blob/master/LICENSE</PackageLicenseUrl>
     <Owners>Xamarin</Owners>
     <PackageProjectUrl>https://github.com/mono/LineEditor</PackageProjectUrl>
-    <PackageReleaseNotes>Release 5.4.1 improves Windows support.</PackageReleaseNotes>
+    <PackageReleaseNotes>Release 5.4.2 improves Windows support.</PackageReleaseNotes>
     <PackageRequireLicenseAcceptance>True</PackageRequireLicenseAcceptance>
     <Summary>An interactive commmand line editor</Summary>
     <Title>Mono.Terminal - LineEdit, GetLine</Title>
@@ -24,7 +24,9 @@
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
     <DocumentationFile>bin\Debug\netstandard2.0\LineEditor.xml</DocumentationFile>
   </PropertyGroup>
+
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
     <DocumentationFile>bin\Release\netstandard2.0\LineEditor.xml</DocumentationFile>
   </PropertyGroup>
+
 </Project>