Fix transparent color

[jefferson-vieira]

thinking about correcting the problem with the transparent color without proposing changes in what had already been defined (such as disable hex4 and hex8), I edited the validation function to pragmatically allow the transparent color, thus avoiding side effects

[jefferson-vieira]

I think this closes #500, #624 and #672

[dsanchez07]

@casesandberg Could you check this PR and merge it if it's fine? It's been a while I need this and would be awesome to have it merged 🙏

[mattandbucs34]

I would also like to have the 'transparent' option available to click. Using transparent hexcode also seems to fail when passing through the isValidHex function.
The issue I am having is that when clicking on transparent, no action seems to happen. It is almost as if the transparent span is inactive.

[casesandberg]

This looks great! Sorry it took me so long to get around to merging it. It will be going live to NPM later today.

Congrats on your first-time contribution 🎉 I am going to be adding you as a collaborator, feel free to accept if you are interested in helping further with the project!

[johnnyfekete]

Hi @casesandberg I see the latest version in the repo is 2.19.0 but on NPM it's 2.18.1

Would it be possible to publish this one? 😊