This PR adds a |preload| arg to the node::LoadEnvironment to allow
embedders to set a preload function for the environment, which will run
after the environment is loaded and before the main script runs.

This is similiar to the --require CLI option, but runs a C++ function,
and can only be set by embedders.

The preload function can be used by embedders to inject scripts before
running the main script, for example:
1. In Electron it is used to initialize the ASAR virtual filesystem,
   inject custom process properties, etc.
2. In VS Code it can be used to reset the module search paths for
   extensions.

Co-authored-by: Joyee Cheung <joyeec9h3@gmail.com>