Fix types

[lcharette]

See : #51 (comment)

With 4.2.0 and Typescript enabled on my project, I now get this error because it doesn't find declarations files:

Could not find a declaration file for module 'limax'.  '[...]/node_modules/limax/lib/limax.mjs' implicitly has an 'any' type.
  There are types at '[...]/node_modules/limax/index.d.ts', but this result could not be resolved when respecting package.json "exports". The 'limax' library may need to update its package.json or typings.

4 import slug from 'limax'

[lovell]

Thank you, I'm going to add a test for this too to help prevent regression.

[lovell]

Definition tests added via commit 5d451e8

v4.2.1 now available, thanks for reporting and fixing.