Tidy imports in the Tact codebase

[jubnzv]

We could add additional yarn rules to sort and report unused imports. For example, this configuration sorts them alphabetically and places local imports above external ones:

https://github.com/nowarp/misti/blob/f95260498be6801beb0364e16ebd8034784d436c/.eslintrc.js#L28-L52

This setup allows ESLint to automatically fix the import order and address some other warnings by running yarn eslint --fix:

https://github.com/nowarp/misti/blob/f95260498be6801beb0364e16ebd8034784d436c/package.json#L32