The ts-proto lib has the huge advantage of producing code the "Typescript" way and not the "Java" way.
In particular, for "oneof" types in protobuf, it is generating "ADT" that can be used to check if we forgot or not to deal with a type.
Turning the "importsNotUsedAsValues" TS config value to "error".
This will require us to use `import type` instead of `import` when we are importing a value that is only used as a type (and therefore that is dropped by the Typescript compiler).
Why this change?
This is a requirement to be able to use Svelte in the future. See https://github.com/sveltejs/svelte-preprocess/issues/206#issuecomment-663193798
