Add role icon properties to ResolvedRole

[sudojunior]

Added

• Role#icon as a nullable string which is an image hash if available including icons as uploaded from custom emojis.
• Role#unicode_emoji as a nullable string which is... a unicode emoji.
• get Role#iconURL -> Role#dynamicIconURL()
• Role#dynamicIconURL(format?, size?) which returns as null if #icon doesn't exist.
• ROLE_ICON(format, size) cdn route to Endpoints

Role#icon and Role#unicode_emoji are reflected from ResolvedRole as it's sibling type definition.

[Snazzah]

These new properties should be exposed in the Role class, along with a roleIconURL and dynamidRoleIconURL method similar to the User class.

[sudojunior]

It should be noted that the new dynamicIconURL method will accept gif as an argument for format, but the CDN will not have that format. I could probably go back in and exclude it from ImageFormat, but then the code logic would also need another change to fallback to a default.