Add or publicly document msbuild variable to control XAML Hot Reload tooling

### Description

Currently XAML Hot Reload is available in Debug config only. It appears to be controlled by [_MauiXamlCValidateOnly](https://github.com/dotnet/maui/blob/main/.nuspec/Microsoft.Maui.Controls.targets#L147). This needs to become public. So that customers can control hot reload experience in their custom configurations. Note that WPF, UWP and WinUI have such msbuild property and it is [documented](https://docs.microsoft.com/en-us/visualstudio/xaml-tools/xaml-hot-reload-troubleshooting):
_By default, source info is included in a Debug configuration. It is controlled by MSBuild properties in your project files (such as *.csproj). For WPF, the property is XamlDebuggingInformation, which must be set to True. For UWP, the property is DisableXbfLineInfo, which must be set to False. For example:
WPF:
True
UWP:
False_

### Public API Changes

`True`

### Intended Use-Case

Custom debug configurations where XAML hot reload support is needed