fix incorrect definition of __all__ in __init__.py file

[Malviyaaadarsh]

Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

Why is this PR needed?

__all__ in brainrender-napari/__init__.py was defined as a string rather than a list. Defining it as a string does not follow standard conventions and lead to import errors when using from brainrender_napari import * in python shell when i tested in terminal. This is much needed for developers using python shell to interact with napari viewer.
Also, the widget classes were not imported at package level, which makes public imports inconsistent.

What does this PR do?

• convert __all__ variable to a list of strings.
• explicitly imports the public widget classes in the __init__.py file.
• Aligns the expected package behaviour with standard practices.

References

None

How has this PR been tested?

Locally verified that widgets imported correctly. Running from brainrender_napari import * in the python shell works without import errors. Launched and tested napari in the shell using napari.Viewer() to ensure plugin behaviour remain the same.

Is this a breaking change?

No.

Does this PR require an update to the documentation?

No.

Checklist:

• The code has been tested locally
• Tests have been added to cover all new functionality (unit & integration)
• The documentation has been updated to reflect any changes
• The code has been formatted with pre-commit