Enable isort-style required-imports enforcement

[charliermarsh]

In isort, this is called add-imports, but I prefer the declarative name.

The idea is that by adding the following to your pyproject.toml, you can ensure that the import is included in all files:

[tool.ruff.isort]
required-imports = ["from __future__ import annotations"]

I mostly reverse-engineered isort's logic for making decisions, though I made some slight tweaks that I think are preferable. A few comments:

• Like isort, we don't enforce this on empty files (like empty __init__.py).
• Like isort, we require that the import is at the top-level.
• isort will skip any docstrings, and any comments on the first three lines (I think, based on testing). Ruff places the import after the last docstring or comment in the file preamble (that is: after the last docstring or comment that comes before the first non-docstring and non-comment).

Resolves #1700.

[charliermarsh]

Two more things here: (1) support straight imports (import os, as opposed to from os import ...), and (2) add tests.

[ashb]

Amazing timing @charliermarsh - I just went looking for this for Airflow