Support for undocumented Bookmark API

[lcheylus]

Twitter launched its "Bookmarks" feature in 2018 (tweets saved privately for a user). Get/save/remove bookmark is undocumented in Twitter API but via Web browser debug, we can found some API endpoints for it => see https://gist.github.com/stepney141/c161a83f02c42e161c905249733b9225

In the same way as for favorites, please add methods to get / save / delete bookmark via API endpoints.

[Harmon758]

Tweepy won't be implementing any undocumented/unsupported/unofficial parts of the Twitter API, as using these endpoints violates Twitter's Terms of Service and Developer Policy.

As for bookmarks specifically, Twitter plans on implementing endpoints to enable creation and consumption of bookmarks, as part of the new v2 API (for support for that, see #1472).

See:
https://twitterdevfeedback.uservoice.com/forums/930250-twitter-api/suggestions/39678766-api-endpoint-for-getting-bookmarks
https://trello.com/c/qztXMWCK/76-bookmarks-v2-twitter-api
https://twittercommunity.com/t/add-api-to-retrieve-and-delete-twitter-bookmark/142091
https://twittercommunity.com/t/bookmarks-api-updates/140498