feat(cli): make extract cmd asynchronous execution

[SergeyPoluektov]

Some custom extractors are required to have async execution. For instance, in order to extract messages from Svelte+Typescript you need to preprocess svelte files before compilation. Preprocessing is an async operation in Svelte (see doc). But lingui-cli extract command doesn't support async extract method from custom extractor – see catalog.make method in packages/cli/src/lingui-extract.ts and all function calls down to extract method of a particular extractor.

This PR adds support for async extractors by awaiting for extract is finished before calling traverse function in Catalog.collect method

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[semoal]

I would love to receive a pull request with a real example of Lingui + Svelte in our monorepo mate, I'm not experienced with Svelte and would love to get Lingui compatible with him.

[SergeyPoluektov]

I would love to receive a pull request with a real example of Lingui + Svelte in our monorepo mate, I'm not experienced with Svelte and would love to get Lingui compatible with him.

Yes, that would be great to have in there. I'll prepare an example with Svelte and a custom extractor.