fix(localstored): mark "engine"-option as optional

[Addono]

The documentation and code mark the "engine"-prop as optional. However, the TypeScript type enforces it to be required, therefore any consumer who tries to omit this prop and use the default built-in engine gets a TypeScript error.

This change updates the argument-type of localstored, such that the "engine"-option becomes optional. Which allows a consumer to use the default build-in LocalStorage-based engine.

Old behaviour:

Documentation examples show usage of this plugin without explicitly setting an implementation for the engine:
https://github.com/avkonst/hookstate/blob/master/docs/index/src/examples/plugin-localstored.tsx

[codecov-commenter]

Codecov Report

Base: 82.61% // Head: 82.61% // No change to project coverage 👍

Coverage data is based on head (fb05433) compared to base (8319bd5).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #372   +/-   ##
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  Coverage   82.61%   82.61%           
=======================================
  Files           2        2           
  Lines         880      880           
  Branches      254      254           
=======================================
  Hits          727      727           
  Misses        120      120           
  Partials       33       33           

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[ssysm]

bump this PR, related to #370 as well.

[avkonst]

Thanks for pr. I am offline on holidays at the moment. I will release once I am back.