Fixed building with node 12.18.2 (latest LTS)

[botanegg]

On fontnik 0.5.3 I got a lot of node-gyp building errors such as

In file included from ../src/glyphs.hpp:5,
                 from ../src/node_fontnik.cpp:2:
...node_modules/nan/nan.h:1044:37: error: cannot convert «char*» to «v8::Isolate*»
 1044 |         length_ = string->WriteUtf8(str_, static_cast<int>(len), 0, flags);
      |                                     ^~~~
      |                                     |
      |                                     char*

Upgrading fontnik to 0.6.0 fixes problem, because it moved to NodeJS N-API