src: preload function for Environment

[zcbenz]

This PR provides an API in node::Environment to allow embedders to set a preload function for 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.

/cc @nodejs/embedders @bpasero