Color ToString formats

[maxkatz6]

Is your feature request related to a problem? Please describe.

Currently, Avalonia Color.Parse supports multiple color formats as an input. But ToString() cannot be configured to reliably output any of them.

Minimum required:

1. Default - well-known const or fallback to XAML hex
2. Well formatted XAML hex format (with forced leading alpha) - #aarrggbb
3. Short XAML hex format (with optional leading alpha) - #aarrggbb, #rrggbb, #argb, #rgb

Optional for CSS compat:

1. Well formatted HTML hex format (with forced trailing alpha) - #rrggbbaa
2. Short HTML hex format (with optional trailing alpha) - #rrggbbaa, #rrggbb, #rgba, #rgb
3. Well formatted absolute CSS rgb - rgb(31, 120, 50, 255)
4. Well formatted relative CSS rgb - rgb(50%, 50%, 50%, 50%)

Note: there is way more css formats that can be supported, but we can't/don't need to support all of them.

Describe the solution you'd like

This proposal suggests implementing standard IFormattable.ToString(string? format, IFormatProvider?).
Similarly to well-known integer/float/date/time formats, there should be a set of formats for colors. IFormatProvider won't be used.

Describe alternatives you've considered

No response

Additional context

Color.ParseExact would be a reasonable proposal on its own. But let's keep it out of the scope.

[maxkatz6]

Use case should be self-explanatory:

HtmlPanel.BaseStylesheet =
    $$"""
    body
    {
        background-color: {{backgroundColor.ToString().Replace("#ff", "#")}};
    }
    *
    {
        color: {{foregroundColor.ToString().Replace("#ff", "#")}} !important;
        font-size: 14px;
    }
    """;

[robloo]

This definitely falls under my domain since I added the new parse formats. I'll get to it if you don't beat me to it.

I have to set aside time in 12 for a number of things so probably over the summer will circle back to things like this.