Add the "use" function, which allows the shell to call code in files

Add the "use" function, which allows the shell to call code in files. Implementation will require some kind of "sub-shell".

See Morel Java: [Interact.use](https://github.com/hydromatic/morel/blob/08cdf268da777f290dfe055c7c2856cf86164728/src/main/java/net/hydromatic/morel/compile/BuiltIn.java#L1337).