Add proposal for simpler multi-targeting conditions

[dsplaisted]

This proposes making conditions for multi-targeted projects simpler. Instead of this:

<SupportedOSPlatformVersion Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'ios'">11.0</SupportedOSPlatformVersion>

You would be able to do this:

<SupportedOSPlatformVersion Condition="'$(TargetPlatformIdentifier)' == 'ios'">11.0</SupportedOSPlatformVersion>

This should be more concise and understandable

Rendered Proposal