[Proposal] MultiMath markup extension based on MultiMathExpressionConverter

[stephenquan]

This is a variation of #2084 which is to provide a markup extension that is a mashup of:

1. MultiBinding
2. MultiMathExpressionConverter
3. expression engine enhancements

Here are some sample usages.

<!-- TextColor rule x0 + x1 + x2 >= 60 ? "Green" : "Red" -->
<Label Text="Meets requirements"
       TextColor="{toolkit:MultiMath {Binding X0},
                                     {Binding X1},
                                     {Binding X2},
                                     Expression='x0 + x1 + x2 &gt; 60 ? &quot;Green&quot; : &quot;Red&quot;'}" />

<!-- Orientation detection : page.Width < page.Height ? "Portrait" : "Landscape" -->
<Label Text="{toolkit:MultiMath {Binding Width, Source={Reference page}},
                                {Binding Height, Source={Reference page}},
                                Expression='x &lt; x1 ? &quot;Portrait&quot; : &quot;Landscape&quot;'}" />

<!-- Square grid : WidthRequest = min(page.Width, Page.Height) -->
<Grid HeightRequest="{Binding WidthRequest, Source={RelativeSource Self}}" 
      WidthRequest="{toolkit:MultiMath {Binding Width, Source={Reference page}},
                                       {Binding Height, Source={Reference page}},
                                       Expression='min(x, x1)'}" >
    <!-- your stuff goes here -->
</Grid>

<!-- Available character count. Change text color to Red when count drops to 0 (zero) -->
<Entry x:Name="entry" MaxLength="10" />
<Label BindingContext="{Reference entry}"
       Text="{toolkit:MultiMath {Binding MaxLength}, {Binding {Text.Length}, Expression='x - x1'}"
       TextColor="{toolkit:MultiMath {Binding MaxLength}, {Binding Text.Length}, Expression='x - x1 ? &quot;Green;&quot; : &quot;Red&quot;'}" />

High-level pseudo-code implementation that shows Bindings and Expression properties:

// MultiMathExtension.cs
[ContentProperty(nameof(Bindings))]
public class MultiMathExtension : IMarkupExtension<BindingBase>
{
    public IList<BindingBase>? Bindings { get; } = new List<BindingBase>();
    public string Expression { get; set; } = "";
    public object ProvideValue(IServiceProvider serviceProvider)
    {
        return (this as IMarkupExtension<BindingBase>).ProvideValue(serviceProvider);
    }
    BindingBase IMarkupExtension<BindingBase>.ProvideValue(IServiceProvider serviceProvider)
    {
        return new MultiBinding()
        {
            Converter = new MultiMathExpressionConverter(),
            ConverterParameter = this.Expression,
            Mode = BindingMode.OneWay,
            Bindings = this.Bindings
        };
    }
}