Unify text colors

Right now, depending on where you are trying to color text throughout the game, there is a good deal of inconsistency:

• Text components' color field can accept a named color or RGB hex string
• Text components' shadow_color field can accept a list of R, G, B, A integers or an ARGB integer
• Team names only support named color input
• The experimental waypoint command can accept a named color or RGB hex string

This proposal is to unify the existing color input methods and introduce some additional accessibility features to the existing named color system:

• Named colors (gold, dark_aqua, etc.) should be registered (namespace can be omitted if it's minecraft, as per usual) and have default ARGB color and shadow representations which could be overridden by resource packs
  
  This allows players with visual impairments (or anyone else for that matter) to change the default colors to fit their needs and would allow servers to introduce consistent theme colors for use across their datapacks/plugins/etc.
• Anywhere that accepts a "text color" from a user should accept a named color, RGB hex string, or ARGB hex string
• The parts of teams/scoreboard that still rely on named color for identification, such as the sidebar display slots and teamKill.<color> compound objective criteria, should instead use team ID to differentiate

These changes would make it clearer as to where colors can be used (a color is a color is a color) and improve accessibility

Thanks!