feat: add store.customData to (de)serialize custom data

[OrbisK]

Description

This PR exports the utoa and atou so that the serialized data can be used outside of the Vue context.

It also adds store.customData, which is serialised too. This should enable playgrounds to persistently store relevant data, such as versions, without the need for separate hash and query parameters.

Additional

I ran pnpm format, which is why there is so much noise in the changes tab. I can revert this if needed. The relevant changes are:

• src/index.ts
• src/store.ts

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