Remove unnecessary field aliases (`Str`, `Int`, `Bool`, etc.)

Marshmallow/Flask-Marshmallow provides some aliases for fields, for example, `Str` and `String`. To keep everything simple and consistent, I suggest removing these aliases, so that the IDE will not prompt `Url` and `URL` when you type "U" (in that case the users have to make the choice each time).

Proposal:

Remove the following aliases:

- Str
- Int
- Bool
- Url
- UrlFor
- AbsoluteUrlFor

Keep the following:

- String
- Integer
- Boolean
- URL
- URLFor
- AbsoluteURLFor

Originally posted in https://github.com/greyli/apiflask/issues/63

Locate the Marshmallow field definitions and references for the listed aliases: Str, Int, Bool, Url, UrlFor, and AbsoluteUrlFor. Review existing tests or documentation covering these names, remove only the aliases proposed in the issue, and verify that the retained names continue to work while the removed names are no longer exposed.

Written by the indexing model from the issue text.