Allow generic type constraints

[rkone]

We should be able to add generic constraints to our typeparam. For example:
@typeparam TEntity where TEntity : IEntity

As of preview3 gives the error "Unexpected literal following the 'typeparam' directive".

[stsrki]

I also think this is a much needed feature.

Any comment on this?

[prosser]

Without this feature, developers will be creating a lot of boilerplate components to workaround C# 8.0 #nullable limitations.

for example, take this overly-simplified .razor:

@typeparam TEnum
@typeparam TContainer
@inherits EnumButtonsComponent<TEnum, TContainer>

@if (Visible)
{
    <div class="btn-group-toggle" data-toggle="buttons">
        @foreach (var option in Options)
        {
            <label class="btn btn-primary @(Equals(Value, option.Value) ? "active" : "")">
                <input type="radio" name="@Name" value="@option.Value" checked="@(Equals(Value, option.Value))" onchange="@(() => Value = option.Value)"/>
                @option.Description
            </label>
        }
    </div>
}

And the class it's inheriting from:

public class EnumButtonsComponent<TEnum, TContainer> : EnumPickerComponentBase<TEnum, TContainer>
        where TEnum : struct
{
}

public class EnumPickerComponentBase<TEnum, TContainer> : AutoLabeledControlBase<TContainer>
    where TEnum : struct
{
    private string? stringValue;

    // without the constraint (or adding #nullable disable), TEnum is indeterminate. You cannot
    // mark it TEnum?, because it could be a value type. Not marking it as nullable raises a warning
    // because it could be null.
    private TEnum value = default;
    // implementation...
}

The EnumPickerComponentBase class needs the constraint due to #nullable limitations (have to pick either nullable or not nullable). Without this feature in Blazor, you would need to have a .razor for each type that you wanted an EnumPickerComponentBase-derived template.