Composition API and Typescript example

[stefnotch]

It'd be lovely to have an example on the Readme that works with the Pinia composition API and with Typescript.

Here's the relevant bit of the Pinia documentation https://pinia.vuejs.org/core-concepts/plugins.html#typescript

[stefnotch]

Adding the share property makes Typescript rather unhappy at the moment.

[wobsoriano]

Should work with latest versions now.