cli: add list-locales flag

[satya-nutella]

Summary

Adds a list-locales flag to print a list of all supported locales and exit

Related Issues/PRs
Closes #12549

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[satya-nutella]

@googlebot I signed it!

[satya-nutella]

@connorjclark should I add some more tests to this? Codecov patch seems to be failing

[brendankenny]

@meehawk the tests for --list-all-audits and --list-trace-categories in index-test.js should give you a good pattern to duplicate, and it makes sense to add a new test for --list-locales alongside those.

We could probably have slightly stricter assertions than those, though: length > 50 maybe, and that the locales include 'en', 'es', 'ru', and 'zh'? (that's the set of locales the intl-pluralrules polyfill checks to ensure Intl support)

[satya-nutella]

@brendankenny that makes sense. I'll try to work on tests for these and push another patch

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[satya-nutella]

@googlebot I consent

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[satya-nutella]

@connorjclark seems to be fixed. Wow! that was weird

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[connorjclark]

Looks good, thanks for the PR!

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