utility: add support for g-prefixed coreutils ls

[belak]

Fixes #1758

Proposed Changes

• Define g-prefixed ls aliases
• Default to g-prefixed ls in aliases

Note that this does change default behavior a bit by changing which ls will be selected for the default alias.

[casaper]

@wadkar

I myself have not installed gnu coreutils yet, because I wasn't aware of this being available on my mac. So you've lifted me to the idea here, thanks! 👍

But seeing this brings me to the idea to look if it'd make sense to do something similar with darwin users having gnu- programmes installed.

I for example, have gnu-sed, gnu-tar and gnu-getopt because I really don't like their standard darwin counterparts.

[belak]

This actually already exists in https://github.com/sorin-ionescu/prezto/tree/master/modules/gnu-utility.

On a similar note, maybe it would make more sense to only change to the g-prefixed tools if gnu-utility is enabled.