feat: option to inject code verbatim

It would be handy for a use case I have for this package to support a feature that could inject code verbatim if it appears in a JavaScript object.

To implement, we could use (A) a special object constructor, (B) a special property on a string, or (C) a tagged template literal returning some special value, which would allow us to detect such cases in the serialization code.

A:

const object = {
  customCode: serialize.verbatim('window.navigator.userAgent')
};

B:

const customCode = 'window.navigator.userAgent';
customCode.verbatim = true;
const object = {
  customCode
};

C:

const customCode = serialize.verbatim`window.navigator.userAgent`;
const object = {
  customCode
};

I tried a hack similar to https://github.com/yahoo/serialize-javascript/issues/32#issuecomment-350184098 using a function as a value and overriding its toString method, but it didn't work since I can't include ( in the code without it being rewritten as part of the function serialization.

Start by reviewing the package's serialization code to understand how JavaScript object values are currently converted and where function strings are rewritten. Decide how the proposed verbatim value should be represented and detected, then verify that injected code remains unchanged while ordinary values retain existing serialization behavior.