fix(plural): Create cache entry for PluralRules created as part of the fallback flow

[nilskaspersson-imtf]

This PR extends the fallback flow discussed in #2252 by creating a cache entry for custom language codes.

This avoids a cache miss that would otherwise happen for every invocation with an invalid language code.

The function is restructured to try...catch only the Intl.PluralRules constructor and assign the created value to a variable in the previous closure. This way, the formatted values and cache entry extension can be reused.

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• tests are included
• commit message and code follows the Developer's Certification of Origin

Checklist (for documentation change)

• only relevant documentation part is changed (make a diff before you submit the PR)
• motivation/reason is provided
• commit message and code follows the Developer's Certification of Origin

[coveralls]

coverage: 95.281% (+0.007%) from 95.274%
when pulling 49f1e93 on nilskaspersson-imtf:2252-plural-rules-cache-custom-language
into 1c04c5d on i18next:master.

[adrai]

thx, it's included in v23.16.8