Update of inflector types and added TypeScript test

lib/natural/distance/index.d.ts

@@ -1,5 +1,8 @@
 /*
-Copyright (c) 2022, Dylan R. E. Moonfire <https://github.com/dmoonfire>, Emily Marigold Klassen <https://github.com/forivall>, Hugo W.L. ter Doest
+Copyright (c) 2023,
+  Dylan R. E. Moonfire <https://github.com/dmoonfire>,
+  Emily Marigold Klassen <https://github.com/forivall>,
+  Hugo W.L. ter Doest
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal
@@ -20,8 +23,6 @@ OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 THE SOFTWARE.
 */
 
-// Based on type definitions on Definitely Typed
-
 declare interface JaroWinklerOptions {
   dj?: number
   /** @default false */

lib/natural/inflectors/index.d.ts

@@ -20,7 +20,14 @@ OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 THE SOFTWARE.
 */
 
-export class NounInflector {
+export class SingularPluralInflector {
+  singularize (token: string): string
+  pluralize (token: string): string
+  addSingular (pattern: RegExp, replacement: string): void
+  addPlural (pattern: RegExp, replacement: string): void
+}
+
+export class NounInflector extends SingularPluralInflector {
   pluralize (token: string): string
   singularize (token: string): string
 }