feat(Stickers): small refractor to be even with the discord-api docs

[ghost]

changes

• as per Adds documentation for sticker_items discord/discord-api-docs#3171 the stickers property on the Message object has been deprecated and replaced with sticker_items.
• deprecated Sticker.*asset properties as currently the asset url fetching is private.
• added check if the sticker is "available" (it's on the discord docs and am curently not aware when this might be the case).

if discord/discord-api-docs#3128 Pull request gets merged in the next days i will update this pull request to support the new sticker packs/zodates.

[cakedan]

Stickers and Sticker Items are two different things, one is a partial object and the other is the full object. I added them both to the beta branch if ya wanna take a look

Adding the depreciated tags would be a good idea for them tho, thank you!

[ghost]

Oh my bad.. i thought that the 0.16.0 was the latest stable branch should have checked it first . I will check the beta branch out!