feat(core): Add CSV import in multiple languages

[heiko-r]

It turned out a little more complex than anticipated. I tried to maintain as much compatibility with the monolingual import as possible, but this required adding an optional mainLanguage parameter to parseProducts, which is used to specify the language to use for the Product.name, Product.slug, etc. properties. So even though the ParsedProduct and ParsedProductVariant interfaces have additional properties now, I think it should be compatible with most, if not all, existing usages. Not sure if you consider this a breaking change.

Also a note: I pushed with --no-verify, because there's an error during e2e testing:

@vendure/core: FAIL e2e/custom-fields.e2e-spec.ts (11.346 s)
@vendure/core:   ● Custom fields › validation › invalid datetime
@vendure/core:     expect(received).toEqual(expected) // deep equality
@vendure/core:     Expected: StringContaining "The custom field 'validateDateTime' value [2019-01-01T05:25:00.000Z] is less than the minimum [2019-01-01T08:30]"
@vendure/core:     Received: "fail is not defined"
@vendure/core:       13 |     };
@vendure/core:       14 | }
@vendure/core:     > 15 |
@vendure/core:          | ^
@vendure/core:       at Object.<anonymous> (utils/assert-throws-with-message.ts:15:33)

It wasn't me! 😬 It was already broken before I made any changes! 😬😉 (and I also didn't investigate it any further yet)

[michaelbromley]

Thank you! I will review this at the beginning of next week.

[michaelbromley]

This is very good! I made a bunch of comments, some of which might then require some changes.

The addition of the mainLanguage arg is not a breaking change since it is optional and I would be very surprised if anyone is using that method directly in their custom code.

Re the "fail is not defined" - I think it may be related to jestjs/jest#11698

[michaelbromley]

Love all that deleted code in the second commit 😄

Thank you for your work on this!