[$300 in SQT] Support .env in mapping handlers

[stwiname]

Currently .env is only supported in ts manifests. Support should be extended to mapping handlers and when building the project it should resolve the appropriate variables.

This requires updating the subql build command and webpack integration to use the .env file for setting variables.

Other requirements

• Documentation should be updated to include the feature
• Changes should have test coverage of 80% on the relevant code.