Adding an option to import well-known types from a provided package.

[nicolasalt]

The user may want to have well-known types in a separate NPM package:

• These generated files are always the same and may feel like they are polluting the codebase.
• For Web, if multiple dependencies of a single project use well-known types, they'll be duplicated in the output Web bundle.

Ideally there would be a single NPM package with well-known types, maintained as a part of protobuf-ts, but this PR is an ok workaround for now.

This option is implemented as an environment variable because the options parser only supports boolean values.