[feature-request] Support for "file|" syntax in set_env

[piotr-kubiak]

From https://tox.wiki/en/latest/config.html:

With my tox.ini as:

[tox]
env_list = py311

[testenv:py311]
allowlist_externals = bash
set_env = 
    file|.env
commands =
    bash -c 'env | grep "FOO"'

and .env:

FOO=BAR

I get

$ tox
py311: commands[0]> bash -c "env | grep \"FOO\""
FOO=BAR
  py311: OK (1.23=setup[0.14]+cmd[1.09] seconds)
  congratulations :) (1.55 seconds)

Which is tottaly expected. However:

$ pre-commit run tox-ini-fmt
tox-ini-fmt..............................................................Failed
- hook id: tox-ini-fmt
- exit code: 1

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "C:\Users\xxx\.cache\pre-commit\reporcnyrb_x\py_env-python3\Scripts\tox-ini-fmt.EXE\__main__.py", line 7, in <module>
  File "C:\Users\xxx\.cache\pre-commit\reporcnyrb_x\py_env-python3\Lib\site-packages\tox_ini_fmt\__main__.py", line 47, in run
    formatted = format_tox_ini(before, opts)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\xxx\.cache\pre-commit\reporcnyrb_x\py_env-python3\Lib\site-packages\tox_ini_fmt\formatter\__init__.py", line 34, in format_tox_ini
    format_test_env(parser, section_name)
  File "C:\Users\xxx\.cache\pre-commit\reporcnyrb_x\py_env-python3\Lib\site-packages\tox_ini_fmt\formatter\test_env.py", line 74, in format_test_env
    fix_and_reorder(parser, name, tox_section_cfg, upgrade)
  File "C:\Users\xxx\.cache\pre-commit\reporcnyrb_x\py_env-python3\Lib\site-packages\tox_ini_fmt\formatter\util.py", line 51, in fix_and_reorder
    section[key] = fix(section[key])
                   ^^^^^^^^^^^^^^^^^
  File "C:\Users\xxx\.cache\pre-commit\reporcnyrb_x\py_env-python3\Lib\site-packages\tox_ini_fmt\formatter\test_env.py", line 107, in to_set_env
    raise RuntimeError(msg)
RuntimeError: invalid line file|.env in setenv