Add stories to leva's .npmignore

[sritchie]

This gives a slight cleanup of the published npm package.

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Latest commit: a576c93

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Name	Status	Preview	Updated
leva	❌ Failed (Inspect)		Jan 7, 2023 at 4:58PM (UTC)

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Latest deployment of this branch, based on commit a576c93:

Sandbox	Source
leva-minimal	Configuration
leva-busy	Configuration
leva-scroll	Configuration
leva-advanced-panels	Configuration
leva-ui	Configuration
leva-theme	Configuration
leva-transient	Configuration
leva-plugin-plot	Configuration
leva-plugin-bezier	Configuration
leva-plugin-spring	Configuration
leva-plugin-dates	Configuration
leva-custom-plugin	Configuration