feat: Add from_hex_str methods to Color and RgbColor

[joshtriplett]

Converting from a 3-digit or 6-digit hex color is a common operation in various formats, including HTML/CSS, SVG, and Git. Add it to anstyle.

I've based this PR on #331, which fixes a bug in this logic.

[epage]

This is pushing further at the boundaries of anstyles original intent which has me hesitant.

The limited scope was for two reasons

• minimize risk of breaking changes so people can trust to put this in their APIs
• minimize compile times so people don't question adding yet another dependency

The main impact I can see this having is more in the "well, you parse X, but what about Y".

Beyond, the question is where do we draw the line?

[joshtriplett]

@epage I originally considered making this pub from anstyle-git, but that felt like the wrong place, since a CSS color parser shouldn't be pulling in anstyle-git just to get a helper function. A one-function anstyle-hex crate seemed gratuitous, though I'll do that if you'd prefer.