Color struct QOL tweak: Preview the built-in colors in-editor

[Demorome]

• Most importantly, added RGBA hex value comments for each built-in color, which shows a preview of the color, at least in VS Code. Only works when the source file is opened, though; it doesn't show up when the summary comments appear when hovering over the static color name.

The rest is pretty minor:

• Specified StructLayout to guarantee it'll be 32-bit packed (probably paranoid).
• Added FromPacked() static constructor.
• Added ToHexString() method.
• Moved all built-in color declarations to their own #region at the bottom.
• Use the byte-overloaded ctor for built-in colors since it has less operations (extremely minor, but while I'm here...).
• Use namespace MoonWorks.Graphics; declaration instead of a scoped declaration to save horizontal viewing space.

[thatcosmonaut]

Overall this is reasonable, just some nitpicks.

[thatcosmonaut]

This is looking good, I'm going to try to split this into a content and a formatting commit.

[thatcosmonaut]

Should we just remove PackedValue()? It's a pretty bad gotcha.

EDIT: I guess it's useful for ImGui interop...

[Demorome]

I was going to rename it to UnsafePackedValue, at the very least.
I'm not sure if it's even useful for ImGui interop, since you can use ImGui.GetColorU32 on the vector-packed form of a Color to get it in the guaranteed format that ImGui will want. Though maybe currently using PackedValue for that just happens to work, I haven't tested it. Still, it's editor code, I don't need it to be that optimal...

[thatcosmonaut]

Then let's remove it.

[Demorome]

Huh, apparently using ImGui.GetColorU32 ensures that your color for ImGui is multiplied by style.Alpha: https://github.com/ocornut/imgui/blob/683f9160b96a2ae419d9a4896481b260e85a1b87/docs/FAQ.md?plain=1#L642
So in a way, using that function should be recommended anyways. Though there is an overload that lets you pass in a uint32.

[Demorome]

I've removed PackedValue(), replacing it with BigEndianPackedValue() in case that functionality would prove useful. It doesn't seem nearly as much of a footgun, at least.

I've also added FromHexString, to complement ToHexString.

Otherwise, the other changes were pretty minor.

[Demorome]

I was thinking of adding unit tests for these functions, using something like https://code.visualstudio.com/docs/csharp/testing, but I felt that adding a whole new project just for this PR would be a bit out of scope. Lemme know if you feel otherwise.

I would probably be using xUnit, since that seems to be the most popular. It would look like: https://xunit.net/docs/getting-started/v3/getting-started