## BREAKING CHANGE
Emitted .js is now es2020, requiring node >= 12.

## Changes
- Use @substrate/dev for build configuration
    - new tsconfig.json targets es2019
    - prettier is now uses hard tabs and 2 spaces, which causes the huge diff
    - some slight type annotations where necessary for claims.ts and method.ts in order to pass the stricter tsconfig
    - Babel is now used to transpile the @polkadot/* packages `.js` from ESM to CJS for jest