Put the standard library under a global directory by default

[janmasrovira]

The projects should reuse some instance of the standard library by default instead of having its own copy.

We can use defaultStdlib identifier in the Package.juvix file to refer to the global stdlib instead of creating a new copy of the stdlib in the project's juvix-build directory.

[vrom911]

I see that the project is using the juvix-stdlib as a submodule. What do you have in mind for this issue? How it should be reused otherwise?

[janmasrovira]

Now that #2000 has been merged, the idea is to put something like the following as the default.

dependencies: 
- $(XDG_CONFIG_HOME)/juvix/<version>/stdlib