Merge `.env` files to the repo root

[ShookLyngs]

Background

Currently, each package/example has an individual .env file with similar requirements for the environment variables. To improve user experience, we can merge these .env files into a single file located at rgbpp-sdk/.env. Furthermore, we can provide a set of default testnet values for the environment variables, so users running the tests for the first time won't have to configure anything.

TODOs

• [ ] Merge .env files and put it in rgbpp-sdk/.env
• [ ] Provide testnet default env variables

Related Discussions

• Developers prefer a development experience that is as close to "zero-config" or "minimal configuration" as possible. https://github.com/ckb-cell/rgbpp-sdk/issues/247