Frontend: Proper gettext extraction with gettext-extractor

What does this MR do?

We currently cannot extract strings that span multiple lines (#42839 (closed)) or if the n__ call spans multiple lines (#47092 (closed)). ruby is just doing regexes on each line instead of using the AST!

Extraction Solution Approach

1. gettext_rails_i18n calls a new script scripts/frontend/extract_gettext_all.js which extracts all externalized strings from *.{js,vue} files. The communication between those scripts is done via JSON. The script is only called once on all files, as all calling once per file has a too big overhead (see below).
2. We are using the package gettext-extractor to extract the externalized strings from javascript files. I have created gettext-extractor-vue which compiles vue files with vue-template-loader to js and passes it on to gettext-extractor

Multiline string handling

Multi line strings are tricky. We are simply replacing newlines with spaces. So

<template>
{{
  __s(`Context|
 This is a nice multi 
  line
 string
 `)
}}
</template>

will turn into:

'Context| This is a nice multi line string'

Performance

I did some performance tests on the scripts:

Duration (h:mm:ss)	Comment
0:01:45	Original timing (baseline)
0:07:19	Calling the script once per file
0:02:07	Calling the script once for all files

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

• Closes #47092 (closed)
• Closes #42839 (closed)
• Closes #48864 (closed)

EE Port

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6829