[XItemStack] Support hex colors

[datatags]

In the XItemStack docs, Color is defined as:

alias Color: for Text "red, green, blue" or "hex color"

However, XItemStack actually doesn't currently parse hex colors, only the comma-separated format is supported. This PR changes that, and now all of the following formats are supported:

• r, g, b (just like before)
• #RRGGBB
• 0xRRGGBB
• decimal representation of r << 16 | g << 8 | b, as used in NBT

Notes

The hex representation is automatically converted to decimal by SnakeYAML (and then converted to a string), so that format and the decimal representation end up being handled the same in the code. The bare format RRGGBB is also supported if it can't be parsed as a base-10 number, but due to that restriction I don't recommend that.

[CryptoMorin]

Thanks!