Add a convinence macro to create a color from a hex code / const function with comptime error

## What problem does this solve or what need does it fill?
Creating a color from a hex code. Alternative to having `Color::hex("...").unwrap()` (or `Srgba::hex("...").unwrap()`)

## What solution would you like?
Either:
- A macro to create a color from a hex code without unwraps, compile error if color is invalid.
- A const function with a compiletime error on invalid color, no need to unwrap

Not sure how this fits in with the new color overhaul :P

## What alternative(s) have you considered?
Leave it as is.

Start by reading the existing Color::hex and Srgba::hex APIs and the color overhaul mentioned in the issue. Check how color parsing and compile-time validation are currently tested. Done means a documented way to construct a color from a valid hex code without unwraps, with invalid input rejected at compile time if the chosen approach supports it.

Written by the indexing model from the issue text.