Add the CoreModule

This is a special module that is *always* imported

For this first version of the module we have three functions:

getAvailableModules() -> string
which returns the list of available pugs modules

getPugsVersion() -> string
getPugsBuildInfo() -> string
which are moved from the 'utils' module

There is no need to use `import core` in the preamble of a pugs script
file.