Formalize Brush string syntax

Description

The string syntax for Brush, used in Xaml through BrushTypeConverter isn't formalised, described, or documented. We have a few tests for it, but that's it. There's no history of this being supported in other XAML platforms, but it is supported in CSS and our syntax is inspired from there.

This document tries to formalise the syntax so it can be used to rewrite the TypeConverter, provide a compiled-to-il version and as a start of a documentation.

Colors(as defined in the ColorTypeConverter) :

hex = "0" | "1" | "2" ... "e" | "f";            (* case-insensitive*)
number = ... ;
percentage = number, "%";
hexcolor = "#",hex,hex,hex(* #rgb *)
         | "#", hex, hex, hex, hex                  (* #argb *)
         | "#", hex, hex, hex, hex, hex, hex          (* #rrggbb *)
         | "#", hex, hex, hex, hex, hex, hex, hex, hex; (* #aarrggbb *)
rgbcolor = "rgb(",number,number,number,")"           (* values outside the 0-255 range will be coerced to that range *)
         | "rgb(",percentage,percentage,percentage")";     (* values outside 0-100% range will be coerced *)
rgbacolor = "rgb(",number,number,number,number")"     (* opacity number will be coerced to 0-1 range *)
          | "rgb(",percentage,percentage,percentage,number")";    (* opacity number will be coerced to 0-1 range *)
hslcolor = "hsl(",number,percentage,percentage,")";     (* h is mod 360, s and l coerced 0-100% *)
hslacolor = "hsla(",number,percentage,percentage,number")";     (* opacity is 0-1 *)
hsvcolor = "hsv(",number,percentage,percentage,")";      (* h is mod 360, s and v coerced 0-100% *)
hsvacolor = "hsva(",number,percentage,percentage,number")";       (* opacity is 0-1 *)
namedcolor = "aliceblue" | "antiquewhite" ... | "yellowgreen"; (*case insensitive*)
color = hexcolor | rgbcolor | rgbacolor | hslcolor | hslacolor | hsvcolor | hsvacolor | namedcolor;

LinearGradient(same syntax as CSS) :

angle = number, ("deg" | "grad" | "rad" | "turn");
side-or-corner = ["left" | "right"] | ["top" | "bottom"];
linear-color-stop = color, [" ", (number | percentage)];
color-stop-list = linear-color-stop, {linear-color-stop};
linear-gradient = "linear-gradient("[(angle | "to" side - or - corners) "," ] , color-stop-list ,")";

radialGradient:

shape = "circle";      (* I don't think we support ellipse *)
position = ["left" | "center" | "right"] , ["top" | "center" | "bottom"] ;
radial-gradient = "radial-gradient(", [[shape | number][" at " position]], color-stop-list ")";

brush:

brushstring = color | linear-gradient | radial-gradient;

Syntax we will stop supporting

We found these syntax in some test and examples. They do not match the CSS one, provide no additional capability and can be confusing (opacity vs color stop):

Color.yellow
Color.pink.50%
#00FFff 40%

(Public) API Changes

While we're at it, we plan to remove public const strings and public nested class from `BrushTypeConverter

dotnet / maui