fix: update useDarkMode to not reset with OS value on every refresh

[refeals]

Fixes https://github.com/juliencrn/usehooks-ts/issues/512

This PR does two things:

1. Update useLocalStorage save initial value

The way things are now, useLocalStorage does not correctly represents the value in the localStorage. It returns a default value even if the key doesn't exist there yet and does not create it, meaning it returns a value that is not necessarily the truth (should be undefined. This causes a problem in useDarkMode (and probably other hooks).

The PR updates the hook to actually save the key/value in localStorage when the hook is called, preserving the hook's initial funcionality.

2. Update useDarkMode to not reset with OS value on every refresh

Removes the useIsomorphicLayoutEffect because it's unnecessary. The value isDarkMode will always be correctly set.