Add support for non-string constants

[rdeago]

I'll let code speak for me:

  <PropertyGroup>

    <!-- This still works, of course -->
    <Constant Name="Greeting" Value="Howdy!" />

    <!-- Constant type can be specified -->
    <Constant Name="Foo" Type="string" Value="Bar" />

    <!-- Int32, Integer (for the sake of VB projects) and int are all valid and map to System.Int32 -->
    <Constant Name="Answer" Type="int" Value="42" />

    <!-- Other numeric types work similarly -->
    <Constant Name="LongAnswer" Type="Int64" Value="42" />

    <!-- DateTime (and maybe TimeSpan, DateOnly, TimeOnly) may be supported too -->
    <Constant Name="BuildTime" Type="DateTime" Value="$([System.DateTime]::Now.ToString("O"))" />

  </PropertyGroup>

If this looks interesting, I can have a PR ready in a few days.

[kzu]

Looks great :)

[jonnyb023]

Yes please :)