feat: add plural form fallbacks

[ajmnz]

What is the purpose of this pull request? (put an "X" next to item)

[ ] Documentation update
[ ] Bug fix
[x] New feature
[ ] Other, please explain:

What changes did you make? (Give an overview)

Added a new configuration option at plurals.fallbackForm to specify a default fallback plural form.

When the source language doesn't utilize all available plural forms, you either have to duplicate keys for the unused forms or risk missing translations when adding a language with additional plural forms.

For example, Catalan uses one and other, while Romanian has one, few, and other. This feature allows setting a fallback form (e.g., other), ensuring that in this case, few resolves to other.

Which issue (if any) does this pull request address?
N/A

Is there anything you'd like reviewers to focus on?
N/A