Add types

[Creskendoll]

Greetings, I've been experimenting with Svelte and this library and thought that types must be added sooner or later, so here it is...

Added .d.ts types complete with docstrings copied from the readme.

Even though defaultChainStore contains the properties listed in the ChainStore, the API is not intended to be used that way. Instead, the store properties are exported separately and not exposed from within DefaultChainStore. The properties are also included in ChainStore so that they can be accessed the way the docs suggest,

Cheers

[clbrge]

Thanks @Creskendoll . I don't use typescript myself so I will trust your judgement on these types.