fix: pluralize nucleus/nuclei correctly

[rlorenzo]

This PR addresses issue #71.

Right now, pluralize('nucleus') doesn't actually pluralize, and a few related forms come back mangled:

pluralize('nucleus')     // 'nucleus'   (expected 'nuclei')
pluralize('nuclei')      // 'nucleis'   (expected 'nuclei')
singularize('nuclei')    // 'nuclei'    (expected 'nucleus')
singularize('nucleuses') // 'nucleuse'  (expected 'nucleus')
singularize('nucleus')   // 'nucleu'    (expected 'nucleus')

After this PR, each of those returns what you'd expect. pluralize('nucleuses') still gives back 'nucleuses' since it's already plural.

How

I followed the same shape as the existing criterion/criteria and genus/genera rules: three anchored regexes (^(nucle)us$, ^(nucle)i$, ^(nucle)uses$) added to regex.singular and regex.plural, with matching skip + replace entries in pluralRules and singularRules. Because the patterns are anchored to the nucle stem, they can't accidentally catch other -us or -i words.

Test plan

• npm test passes (15 existing tests, plus 6 new assertions covering every case the issue mentioned)

[rlorenzo]

@p-kuen The merge conflict was resolved.