Initial nodejs string wrap

[MarkReedZ]

I've created a Str object wrapper for Node.js

    const a = new stringzilla.Str('hello world');
    const b = new stringzilla.Str('hi');

    const result_1 = a.count(b);

And implemented a few functions with tests.

        {"indexOf", NULL, str_find, NULL, NULL, NULL, napi_default, NULL},
        {"find", NULL, str_find, NULL, NULL, NULL, napi_default, NULL},
        {"rfind", NULL, str_rfind, NULL, NULL, NULL, napi_default, NULL},
        {"startswith", NULL, str_startswith, NULL, NULL, NULL, napi_default, NULL},
        {"endswith", NULL, str_endswith, NULL, NULL, NULL, napi_default, NULL},
        {"count", NULL, str_count, NULL, NULL, NULL, napi_default, NULL}