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.

[timostamm]

Nikolay, I agree that the best way to handle the well-known types is to publish them to a package. This is one of the improvements of protobuf-es. You can still generate them explicitly if you want, but by default, well-known types are imported from a package on npmjs.com. The plan is to switch to the base types provided by protobuf-es (with a small compatibility layer), so we will get that feature for free*.

Apologies for closing this, but I the handling of WKT in protobuf-es already addresses all complications, and we're better off building on top of it instead of duplicating the effort.

• not actually for free, because a nice migration path is a bunch of work 😄