Use alias _App for App in type hints under TYPE_CHECKING

[uriyyo]

Fixes #6158

Hi,

First of all, thanks for such a great library, really enjoy using it 🙌

I have found small issue. App imported in __init__ of textual, but it imported only in if TYPE_CHECKING block, because of that mypy and other tools think that from textual import App is valid code.

from textual import App

$ mypy example.py
Success: no issues found in 1 source file

This PR will import App as _App, so IDEs and other tools will show an error for:

from textual import App

Please review the following checklist.

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

[willmcgugan]

Thanks. This feels like something the linters should understand. It's also a common pattern.