This commit adds associated type to "NodeEnv" trait, called "Fs".
The "Fs" type has a trait bound on "NodeFs", which specifies APIs
required for all ops and resolution APIs to function.
A "RealFs" implementation of "NodeFs" is exported from the "deno_node"
crate, that provides a default implementation for the trait.
All code in "deno_node" extension was changed to use the "NodeFs" trait
to handle file system operations, instead of relying on APIs from the
standard library.
