Prevent useSnackbar from returning a new object on every call

[DASPRiD]

At the moment, useSnackbar will return a new object on every call.

This causes a problem when using it in an effect and specifying it as a dependency (exhaustive-deps check will fail otherwise), as it will cause the effect to run every time the component renders, since React sees the snackbar object as changed.

By returning the context directly, the object should never change.

[iamhosseindhv]

@DASPRiD Would you be able to provide a simple reproduction with console log showing useEffect being called on every render.

[DASPRiD]

I'm currently on vacation, will look into that after Feb 25.

[DASPRiD]

Here is a simple test case:
https://codesandbox.io/s/focused-hodgkin-evq3u

By clicking the button, it causes the app to rerender. This in turn will, without my patch, cause the effect to rerun (see console).

[iamhosseindhv]

Sorry I messed up. my commit to fix a linting issue was supposed to be part of this PR, so github counts you as contributor of the repo.

Anyhow, Thanks for your contribution ❤️I'll publish the fix as v0.9.9