src: remove const qualifier on Object::GetPropertyNames and Object::InstanceOf

[legendecas]

These operations can call into JavaScript and result in arbitrary
side effects, hence shall not be considered as const member functions.

E.g. the following examples intercepts the operations by Proxy handlers:

const obj = new Proxy({
  foo: 0,
}, {
  ownKeys(target) {
    target.foo++;
    return [ 'foo' ];
  },
  getPrototypeOf(target) {
    target.foo++;
    return Object.prototype;
  },
});

Object.getOwnPropertyNames(obj);
obj.foo // => 1
obj instanceof Object;
obj.foo // => 2

[mhdawson]

@legendecas we might need to consider this breaking, as code may no longer compile...

[mhdawson]

There has been some discussion as to what const means. Maybe we should discuss in the next team meeting.

[legendecas]

@mhdawson Yes, this is a breaking change. Apart from the const semantics, do we have any policies on node-addon-api like semver-major changes or breaking changes?

[legendecas]

Labeled as "do not land" until we concluded with a policy regarding the breaking changes and find a consensus on const qualifier semantics on node-addon-api.

[legendecas]

Discussed at today's node-api meeting. This is quite similar to const pointers and pointer to const objects. These functions did not change the Napi::* c++ objects so it seems reasonable to keep them const.