Add support for non-string constants

[kzu]

Discussed in #103

^{Originally posted by rdeago May 19, 2022}
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]

Hopefully now that we have incremental generators everywhere, @rdeago can send a PR? 💯

[justinwritescode]

Can't have a const DateTime....it's dumb, but that's the way it is!

[kzu]

Yeah, the generator should be smart about what can be emitted as a constant vs static read only

[kzu]

I guess I'll have to work on this myself, since @rdeago hasn't sent a PR yet 😉