Bash alias command substitution window resize bug

[lap1nou]

Please review the following checklist.

• Docstrings on all new or modified functions / classes
• Updated documentation
• Updated CHANGELOG.md (where appropriate)

Greetings,

I'm making this PR following a conversation with @xavierog (thanks to him) on Discord. I asked him for help regarding my TUI application.

My application was not resized correctly, the display was bugged and for example the Footer was disappearing, the thing is that I was running the Python application from a bash alias using command substitution (alias cmd='$(python3 tui-app.py)'), @xavierog discovered that the Textual driver was using shutil.get_terminal_size() to get the window size, but in that specific case, the function is failling because the command is ran from a bash command substitution.

This PR aim to fix that specific (and probably uncommon) situation, I ran all tests and here is the results:

================================== short test summary info ==================================
FAILED tests/snapshot_tests/test_snapshots.py::test_text_area_themes[css] - AssertionError: assert False
FAILED tests/snapshot_tests/test_snapshots.py::test_text_area_themes[monokai] - AssertionError: assert False
FAILED tests/snapshot_tests/test_snapshots.py::test_text_area_themes[dracula] - AssertionError: assert False
FAILED tests/snapshot_tests/test_snapshots.py::test_text_area_themes[vscode_dark] - AssertionError: assert False
FAILED tests/snapshot_tests/test_snapshots.py::test_text_area_themes[github_light] - AssertionError: assert False
FAILED tests/snapshot_tests/test_snapshots.py::test_text_area_wrapping_and_folding - AssertionError: assert False
FAILED tests/text_area/test_languages.py::test_setting_unknown_language - Failed: DID NOT RAISE <class 'textual.widgets._text_area.LanguageDoesNotExist'>
FAILED tests/text_area/test_languages.py::test_register_language - ImportError: cannot import name 'get_language' from 'textual._tree_sitter' (/root/textua...
FAILED tests/text_area/test_languages.py::test_update_highlight_query - AssertionError: assert 0 > 0
======== 9 failed, 3104 passed, 2 skipped, 4 xfailed, 1 warning in 259.55s (0:04:19) ========

I had the same results with a fresh Textual clone so I don't think my changes broke any tests.

[xavierog]

@lap1nou: Looks good to me.

Questions for @willmcgugan and @darrenburns:

1. What do you think re: the issue itself and the suggested fix?
2. Beyond that fix: here, we have clear code duplication, calling for refactoring: either introducing an AbstractLinuxDriver class or turning that method into a standalone function with an fd argument that would go into e.g. drivers/_tty.py -- is this worth changing here?
3. What about HeadlessDriver? I think it could be stripped down to:

    def _get_terminal_size(self) -> tuple[int, int]:
        return (80, 25) if self._size is None else self._size