Add polkadart to interact with any polkadot-based blockchain

[leonardocustodio]

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Description and links to repo and pubdev if any

Polkadart is the first dart/flutter package that enables developers to interact with any Polkadot-based blockchain. It is a fully-featured client that includes signing transactions, decoding data, querying the storage, code generation, RPC connections, etc.

https://github.com/leonardocustodio/polkadart

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Checklist

• [x] I read How to contribute
• [x] I edited the SOURCE.md file only
• [x] Added a link to the repo in the PR

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