feat(PerststedState): Add .reset, remove key if set to undefined

[xl0]

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[xl0]

I feel this makes the code more explicit an readeable when I want to revert to the default state.
Should also help with #388

[TGlide]

Thanks for the PR! #388 has since been closed, but this could be useful.

however one thing that I'm thinking about... @huntabyte we probably should allow setting the state as undefined, no? as in to allow any type of data inside localStorage, instead of being picky about undefined, specially since we allow null

[huntabyte]

I want to say there was a reason it is the way it is. Will look into the code later to see.

[xl0]

For my usecase, I can live with just .reset(). Want me to update the PR to limit to .reset() without touching undefined?