refactor: switch to async/await syntax (#47)

Refactor manual promise chaining to `async`/`await` syntax for
readability and maintainability. This should be a completely transparent
change, except that errors thrown by the loader may look slightly
different than before, as they're no longer being constructed manually
within `catch` blocks/callbacks.

Note that per https://node.green/#ES2017-features-async-functions,
`async`/`await` syntax has been supported natively since Node 7.10,
which is below this package's specified minimum version (12), as well as
the specified minimum versions of both webpack and protobufjs.
Therefore, this isn't being treated as a breaking change.