update deprecated use of package.json main entry

Was getting a deprecation warning related to the package.json main entry, and apparently 'dist/' should no longer be used.

Deprecation message:

(node:57062) [DEP0128] DeprecationWarning: Invalid 'main' field in '.../node_modules/fullwidth-halfwidth-converter/package.json' of 'dist/index.js'. Please either fix that or report it to the module author