`singularize()` returns non-words for some regular `-se` nouns (e.g. `gases` → `gase`)

[fedemelo]

inflection.singularize() returns incorrect, non-word results for a small set of common nouns whose plural ends in -ses. The stripped form is not a real word, and singularize(pluralize(x)) !== x.

To reproduce (inflection@3.0.2):

const inflection = require('inflection');

inflection.singularize('gases');   // → 'gase'   (expected 'gas')
inflection.singularize('lenses');  // → 'lense'  (expected 'lens')