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.

[epage]

I understand that it would be nice for this to live someplace and there not being a clear home for it. However, after thinking on this more, anstyle is not that place. As I work to add other anstyle crates as dependencies of other packages, it reminds me of the importance of keeping scope narrowly focused so it is easier to convince people to pull in an extra dependency.

As a stop gap, i would be fine having something like this in anstyle-git. While has a more specific purpose and could feel weird to pull in just for this, it is likely to be a small number of users in this situation that will have to deal with this and, as you pointed out, it already has to have a parser for this.