Create a (Discord)Color using a string

[Adrigorithm]

Description

It allows for Colours to be created from strings using constructor overloading. Please accept my first PRussy to Discord.Net :^)

Changes

• new constructor taking a string (and an enum for possible future color formats))
• tests for this constructor

[Misha-133]

Since it's parsing a string value, the logic should go into a static Parse method
Could you do that? And also make a bool TryParse() one please

[Adrigorithm]

Added the bitmasking and uh the if (rawValue.StartsWith("0x")) was needed because people can actually create hex codes that include leading 0's :(

[itslychee]