feat: add the ability to extract concatenated comments

[tannerlyons]

Hello! Once again thanks for lingui. We're using it more and more at Smartsheet.

I ran into this after my translation comments were quite verbose. I wanted to break them into multiple lines and concatenate them:

...
comment: "first long line" +
"second long line",
...

1 question, if you'd like to go forward with this PR: should this concatenation extraction be applied to the id as well?

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[tannerlyons]

Howdy, I don't understand the following error in the pipeline as it relates to my changes: https://github.com/lingui/js-lingui/runs/4014369422?check_suite_focus=true#step:3:40

Any advice on how I can go about solving this?

...
$ node ./scripts/build/
...
- @lingui/react.development.js (node_dev)
✖ @lingui/react.development.js (node_dev)
-- PLUGIN_ERROR (rpt2) --
packages/react/src/I18nProvider.tsx:2:22 - error TS6059: File '/home/runner/work/js-lingui/js-lingui/packages/core/src/index.ts' is not under 'rootDir' '/home/runner/work/js-lingui/js-lingui/packages/react/src'. 'rootDir' is expected to contain all source files.

2 import { I18n } from "@lingui/core"
                       ~~~~~~~~~~~~~~

error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

[semoal]

Is not your problem Tanner don't worry, the pr looks good to me mate. Will fix the CI asap

[semoal]

Thanks mate, lgtm