This commit adds automatic injection of Node globals when "--compat" flag
is present.
This is done by executing "https://deno.land/std/node/global.ts" as a "side module",
before main module is executed.
This commit makes "--compat" required to be used with "--unstable" flag, as some
of Node globals require unstable Deno APIs.
This commit adds "--compat" flag. When the flag is passed a set of mappings for
built-in Node modules is injected into the import map. If user doesn't
explicitly provide an import map (using "--import-map" flag) then a map is
created on the fly. If there are already existing mappings in import map that
would clash with built-in Node modules a set of diagnostics is printed to the
terminal with suggestions how to proceed.
