feat(persist-state): 🔥 Add callback to change state before save

[johnfewell]

The preStorageUpdate feature in the persistState function allows you to modify the state of your application before it is saved to storage. This is particularly useful when you want to remove or transform certain parts of the state before persisting it. For example, you might want to remove sensitive data or unnecessary information. The function receives the store's name and the current state as parameters and should return the modified state which will then be saved to storage.

✅ Closes: 490

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngneat/elf/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
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PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

There is no option to update the state before saving to storage.

Issue Number: 490

What is the new behavior?

There is an optional callback to update the state before saving to storage called preStorageUpdate.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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