Restructure the plugin to rely on Starknet.js

Originally, the Hardhat Plugin was designed as a wrapper around the official Starknet CLI, since back then that was the only option to reliably interact with the network.

Now, Starknet CLI will be deprecated soon, and also Starknet.js is mature enough to replace all of the functionalities of the CLI.

This will be a major change, a lot of code could be deleted (and open issues closed). Maintenance will also be a lot easier.

This is actually the way Hardhat works on Ethereum - it relies on Ethers.js for interactions with the Ethereum blockchain, so the community should not be surprised with this change.

A migration guide should also be written.

The plugin currently wraps the Starknet CLI; the task is to replace that with Starknet.js. Start by examining the plugin's source to understand the CLI interactions being wrapped. Look at how Hardhat's Ethereum plugin uses Ethers.js as a reference. A migration guide must be written for users. 'Done' means the plugin uses Starknet.js for all network interactions and the old CLI code is removed.

索引モデルが issue の本文から書いたものです。